In recent weeks, several of the world’s leading consumer brands, including Kelloggs and Allstate, announced that they will no longer buy advertising on Breitbart.com, a news and commentary site that is popular among American conservatives and whose former CEO, Steve Bannon, is a top aide to President-elect Donald Trump.

It is an issue that should concern anyone who believes in the importance of independent journalism. The internet is a powerful platform for debate and deliberation, and advertising is its most important source of funding. When media organisations lose their access to advertising revenue because they voice provocative or unpopular opinions, it impoverishes the marketplace of ideas.

I should know. I am the CEO of the world’s second-largest advertising technology company. And last month, I made the difficult decision to remove Breitbart’s inventory from our exchange. Let me explain why.

Most digital advertising marketplaces impose quality controls. Just as most brick-and-mortar retailers will not sell hardcore pornography or stolen goods, ad exchanges impose restrictions on the inventory that is sold with their technology. For instance, three years ago, amid a battle over music piracy, several companies, including mine, unilaterally stopped serving ads on sites supported by piracy. The reason was simple. We regard piracy of music and film as an infringement of intellectual property, and we won’t be party to such activity.

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Another good reason to enforce quality controls is “brand safety.” Most Fortune 500 advertisers are loath to see their campaigns appear next to – or embedded in – graphically violent content. As a growing share of digital advertising inventory is bought and sold by real-time auction, rather than through hand-sold processes, advertisers rely on their technology partners to screen out such content, or to give them tools to screen it out themselves. As a spokesperson for Kellogg’s explained with regard to Breitbart, “We regularly work with our media buying partners to ensure that ads do not appear on sites that are not aligned with our values as a company.”

Which brings us back to Breitbart. Last month, after careful consideration, my company decided to sharpen its policy on hate speech. Our customer service policies have long included a prohibition against hateful and offensive content, but identifying such content is, of course, a tricky endeavour.

While every human being is entitled to his or her opinion, speech also has ability to cause harm and incite violence. Susan Benesch, a faculty associate at the Berkman Center for Internet and Society at Harvard University, has studied dangerous speech patterns that result in outbreaks of mass violence. She draws an important line between speech that is repugnant or controversial and speech that catalyses violence. Using her guidelines, we determined that Breitbart exhibits a pattern of speech that explicitly incites violence against various minority groups.

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Our industry – internet advertising – sits at the intersection of commerce and discourse. The internet is in many respects a global public utility. It should belong to everyone. But when someone uses that utility in a manner that may incite violence or discrimination, we also have a responsibility to turn off the mains.

This imperative isn’t unique to the United States. Europe is also grappling with the rise of extremist hate groups that target minority communities and individuals.

That said, a healthy political culture relies on free discourse. With critical elections scheduled next year in France, Germany, and the Netherlands, there is more need – not less – for a vibrant and open internet.

Striking this balance is difficult. Ad tech companies shouldn’t presume to be the internet’s thought police. But they also should not help peddlers of hate monetise violent incitement.

I hope that Breitbart will remove or revise hateful articles that may incite violence and enforce higher standards for its content. Once they do, we will welcome them back to our exchange. Many of my colleagues won’t like it, but that is what it means to operate in an open democracy.

In the meantime, if you are a major brand marketer, pull your budgets from any publisher that supports hate speech or piracy. If you are a publisher, establish and enforce policies that prevent dangerous speech.

I love the internet, and I am willing to take difficult actions to protect it. I hope you will join me in keeping the internet open, safe, and vibrant.