President Obama is scheduled to announce this evening his much rumored-about plan to protect several million undocumented immigrants from deportation. I have explained in several places why this plan does not violate the Constitution, and is indeed a routine exercise of the president’s enforcement authority under the Constitution, one that his predecessors have used as well. But it’s certainly jarring to many people who ask, reasonably: If the president can refuse to enforce immigration law, what stops a future president from refusing to enforce tax, environmental, or health laws? Immigration law is special, and it is worth explaining why.

America has a huge and insatiable hunger for cheap labor—workers to mind the kids, trim the hedges, pick strawberries, and slaughter chickens. But the United States also has numerous laws that make labor expensive. These laws impose minimum wages and maximum hours, give workers the right to unionize, and protect them from unsafe conditions. They also provide welfare to those who don’t work, and many people prefer no job to a menial job. The result is that few American workers will do the really cheap labor that so many households, factories, and farms demand.

Foreigners, however, will. In Mexico, 40 million people earn less than $2,000 a year. They can migrate to the United States and earn 10 or 15 times that amount even if they work off the books. True, they cannot form unions or complain if the workplace is unsafe, but life is still better than back home. The laws that matter are the laws of supply and demand, as a result of which 11 million people reside illegally in the United States.

Many people who feel threatened by legal immigration have been able to live with this system of illegal labor. Because undocumented immigrants are denied social services, Americans don’t pay taxes to support them. Because they do menial jobs, they don’t undercut the wages of most American workers. Because they can’t vote, they can’t get these rules changed through political action.

This “illegal immigration system” might seem to be mutually beneficial—they get jobs, we get cheap labor—but it is unstable. The people who come to work here for cheap wages often settle permanently and become integrated in communities that include American citizens. They intermarry or they arrive as an American’s parent, sibling, or child. The natural sort of sympathy toward the laboring poor that animated many of the protective laws for Americans has led to political pressure to extend the laws to undocumented immigrants as well. People feel uneasy that a large group of second-class citizens resides on our soil. Hence the constant drumbeat from many quarters for a pathway to citizenship.

But to give undocumented immigrants citizenship is to acknowledge that they are entitled to it, and that the “illegal immigration system” is unjust. The current system violates deeply ingrained American principles, which hold that everyone should receive equal protection of the law. That is why the obvious solution to illegal immigration—a lawful guest-worker system—is opposed by nearly everyone, but especially liberals, who see it as institutionalizing a caste system. Indeed, countries that use formal guest-worker systems—like the Persian Gulf countries—are routinely accused of exploiting and abusing migrant workers, of maintaining a caste system or even a system of de facto slavery, of violating human rights law, even though those workers benefit massively from wages much higher than they could earn at home.

The contradiction between ideological opposition to guest workers and the huge demand for cheap foreign labor is the key to the present controversy. To avoid the appearance of a legally recognized caste system while allowing one to exist in reality, Congress has given nearly full legal rights to legal immigrants and passed tough laws to keep everyone else out—while appropriating far too little money to enforce them. This throws to the executive the task of deciding whom to enforce the laws against. Because Congress appropriates only enough money to deport 400,000 people per year out of 11 million, the president by necessity must pick and choose whom to deport. It’s no surprise that for decades every president has deported mainly criminals while leaving most everyone else alone.

For example, in 2014, immigration authorities deported 368,644 people, which may sound like a lot. But 98 percent of them were criminals, were caught just after entering, or had engaged in various serious forms of immigration fraud. The probability of a noncriminal undocumented alien living in the interior being deported is less than 1 in 1,000.

Nor does the government put much pressure on employers. In 2012, immigration authorities fined only 495 employers for illegally hiring undocumented aliens; the aggregate fines amounted only to $12.5 million, a pittance when you consider that 8 million unauthorized migrants are employed. (Republicans might be interested to know that zero employers were fined in 2006 and two employers were fined in 2007, under the Bush administration.)

Thus, the president’s discretion to enforce the immigration laws has always been the cornerstone of a de facto guest-worker (or, if you want, caste) system from which most Americans have greatly benefited. That’s why Republicans’ claim that the president is shredding the Constitution sounds so odd to people knowledgeable about immigration law. He’s just doing what countless Congresses have wanted him to do, and have effectively forced him to do, so that Congress itself could avoid charges that it has created a two-tier system of citizenship where the bottom tier is allowed to stay in this country and work, but is not allowed to vote, to benefit from welfare programs, to travel freely, or to enjoy the full protection of workplace laws. Of course, you might say that the whole illegal immigration system, with its two-tier system of rights, violates the Constitution or at least constitutional values, but the fault for that lies with Congress, not with the president.

But Republicans are right about one thing. Obama’s action will not fix the problem of illegal immigration; nor would congressional action that created a legal pathway to citizenship. The great irony is that as undocumented aliens gain rights, they will no longer need to, or even be able to, supply menial work at a low wage. Illegal immigration will rise again, just as it did after the last path-to-citizenship-law in 1986. America’s hunger for cheap labor can’t be legislated away.