In response to NFL player Colin Kaepernick’s recent protests about police abusing their power, the Santa Clara police union over the weekend threatened to stop providing security at the stadium where Kaepernick’s San Francisco 49ers play.

In a letter sent to the 49ers management Friday, the Santa Clara Police Officers’ Association acknowledged that Kaepernick “exercise[d] his right of free expression” when he refused to stand for the national anthem to bring attention to racial disparities in police shootings. But the union added that it has a “duty to protect its members and work to make all of their working environments free of harassing behavior,” then threatened to stop offering security at Levi’s Stadium in Santa Clara, California.

Police officers voluntarily agree to work at Levi’s Stadium. If enough officers don’t sign up for the event, the local police chief can assign officers to a game at the stadium.

In a statement to USA Today, Santa Clara Police Chief Michael Sellers rebuked the letter: “I will urge the [Santa Clara Police Officers' Association] leadership to put the safety of our citizens first. I will work with both sides to find a solution. In the meantime, I will ensure we continue to provide a safe environment at Levi’s Stadium.”

“Many of us in the law enforcement community have been saddened and angered by Kaepernick's words and actions,” Sellers said. “His blanket statements disparaging the law enforcement profession are hurtful and do not help bring the country together. As distasteful as his actions are, these actions are protected by the Constitution. Police officers are here to protect the rights of every person, even if we disagree with their position.”

Kaepernick grabbed headlines over the past several weeks after he refused to stand for the national anthem before preseason games and later wore socks with cartoon pigs in police outfits. He’s said that “there is police brutality. People of color have been targeted by police.” He specifically called out the San Francisco Police Department, arguing, “We have cops in the SFPD that are blatantly racist and those issues need to be addressed.” (The SFPD has been mired by scandals involving racist text messages over the past couple of years, including several “jokes” that repeatedly use the n-word.)

The police union’s boycott threats essentially prove Kaepernick’s argument. As Adam Stites pointed out at SB Nation, “Kaepernick’s message boils down to police being selective about who they choose to serve and protect. Officers threatening to pull their services from 49ers games because of his comments essentially validates them.”

What’s more, there really are racial disparities in police forces — even though police unions across the country have repeatedly protested such claims from the Black Lives Matter movement over the past several years.

There are big racial disparities in how police use force

It is obviously true, as the police union points out in its letter, that not all cops are bad or racist. But the problem is not whether some individual police officers can do a good job. The key issue here is whether there are systemic racial disparities in how police use force.

The data certainly shows there are very big disparities. An analysis of the available FBI data by Vox’s Dara Lind shows that US police kill black people at disproportionate rates: They accounted for 31 percent of police killing victims in 2012, even though they made up just 13 percent of the US population.

A 2015 study by researcher Cody Ross that looked at police shooting data also found, “There is no relationship between county-level racial bias in police shootings and crime rates (even race-specific crime rates), meaning that the racial bias observed in police shootings in this data set is not explainable as a response to local-level crime rates.”

There are many systemic issues at play here: how police are disproportionately deployed in minority neighborhoods, how cops are encouraged to make as many arrests and issue as many tickets as possible, how they — like much of the public — hold subconscious biases against black people.

Some police officers have acknowledged these issues. Some New York City officers said that they actively target minority communities, for example, to meet informal quotas for stops and arrests effectively set in place by higher-ups.

“When you put any type of numbers on a police officer to perform, we are going to go to the most vulnerable,” Adhyl Polanco, a New York City police officer, told New York’s WNBC. “We’re going to [the] LGBT community, we’re going to the black community, we’re going to go to those people that have no boat, that have no power.”

This is what Kaepernick is protesting: not individual police officers but a system that pushes even good cops to do the wrong thing and potentially act in racist ways.

Police unions are often the most ardent defenders of police

While Kaepernick’s protest is relatively new, the attitude behind the police union’s letter is not — time and time again, police unions have been some of the biggest, most outspoken defenders of police.

For example, in 2014, Cleveland Browns player Andrew Hawkins wore a shirt calling for “Justice for Tamir Rice and John Crawford,” both of whom police in Ohio shot and killed after reportedly mistaking toy guns they were holding for real firearms.

Then–Cleveland Police Patrolman Union president Jeff Follmer quickly fired back in a statement to local news station NewsNet5, calling Hawkins's actions “pretty pathetic” and demanding an apology from the Browns. When Hawkins refused to apologize, Follmer told MSNBC's Ari Melber that the Rice shooting — in which a 12-year-old boy died after a police officer shot him within two seconds of getting out of his squad car — was “justified.” Follmer also suggested Rice’s death was his own fault.

“If you’re wrong, you’re wrong. If you’re right, then the courts will figure it out.”

“I think the nation needs to realize that when we tell you to do something, do it,” he said. “If you’re wrong, you’re wrong. If you’re right, then the courts will figure it out.”

To some degree, this is police unions merely filling their roles: Unions are required under the “duty of fair representation” covered by the National Labor Relations Act and state laws to give the best possible protections — including legal aid and support in job negotiations — to all their members.

But they’re not legally required to use inflammatory rhetoric, like effectively blaming a 12-year-old for his own death, or to get into public fights with NFL players and their teams.

The 49ers organization, for its part, isn’t backing down so far. In statements, the team has said they respect Kaepernick’s right to protest: “In respecting such American principles as freedom of religion and freedom of expression, we recognize the right of an individual to choose to participate, or not, in our celebration of the national anthem.”