TWO YEARS ago, confronted with an inflammatory advertisement depicting the prophet Muhammad, the agency that runs Metro banned all “issue-oriented advertising” from the subway and bus systems. This March, the U.S. District Court for the District of Columbia found the ban constitutional. You might expect this to be the end of the story — but it turns out that defining “issue-oriented advertising” isn’t quite as simple as it sounds.

The Washington Metropolitan Area Transit Authority is facing a new First Amendment lawsuit from the American Civil Liberties Union. And the ACLU has a point — at least in part. While WMATA’s policy bans advertisements advocating for any side of any issue, it has determined what counts as advocacy in a manner that privileges some viewpoints over others. And its guidelines are so vague that it’s hard to say what WMATA considers advocacy to begin with.

The ACLU is suing on its own behalf — Metro rejected its effort to display the text of the First Amendment — and on behalf of far-right provocateur Milo Yiannopoulos, the Carafem abortion clinic and People for the Ethical Treatment of Animals, all of whose advertisements WMATA rejected. In Mr. Yiannopoulos’s case, WMATA approved the advertisement only to remove it after receiving complaints from offended riders.

The ACLU’s suit goes too far in arguing for WMATA to accept its advertisements and those of PETA, which encouraged veganism. Those really were issue ads prohibited by WMATA’s legally acceptable guidelines.

But Mr. Yiannopoulos’s advertisement aimed not to broadcast a viewpoint but to sell his book. WMATA appears to have rejected the advertisement based on complaints about Mr. Yiannopoulos’s politics, when it does accept ads for other creative works; that amounts to the government’s unconstitutionally selecting which ads to display on the basis of viewpoint. Likewise, WMATA rejected Carafem’s advertisement promoting the clinic’s services as advocacy because the ad implicated the abortion debate. The ACLU makes a persuasive argument that WMATA’s choice to label all abortion-related advertisements as issue-oriented constitutes viewpoint discrimination as well.

WMATA must apply its guidelines consistently, even to products associated with contentious issues. It should also provide clear, objective guidance as to what constitutes advocacy. Of course, even advertisements for products often put forward a point of view to some extent. The vice president of Carafem, for example, has stated that the clinic hopes its ads will increase abortion’s social acceptability. A McDonald’s ad might promote the consumption of meat.

Nevertheless, we can distinguish between advertisements that primarily promote products and those that promote only ideas. Legally, WMATA can prohibit the latter. But if it allows the former, it should approve advertisements for all products and services that meet WMATA’s other guidelines, no matter how controversial the views behind those products may be. And while some ACLU supporters — even one of its own attorneys — have criticized the organization for representing Mr. Yiannopoulos, we should celebrate its willingness to remind us that the First Amendment also protects those voices we may find loathsome.