Asian-American band The Slants (L-R: Tyler Chen [drummer], Ken Shima [singer], Simon Tam [bassist)] Joe Jiang [guitarist]) in Old Town Chinatown, Portland, Ore., in August 2015. (Anthony Pidgeon/Anthony Pidgeon)

A DECENT business owner would not, or should not, use a straightforward racial slur as a trademark. In a nutshell, that’s why we have called upon Washington’s pro football team to abandon its offensive nickname. Whether business owners who think differently have the same right to federal intellectual property protection as everyone else is a difficult constitutional question, however, which the Supreme Court is now pondering after hearing oral arguments on Jan. 18. We don’t see how the justices can say anything but a qualified “yes,” despite the possibly favorable legal precedent that would set for the football team, whose own appeal of adverse rulings in a separate case is still pending.

Trademarks enjoy various forms of protection under state and federal law, but those that obtain federal registration get valuable extra benefits, such as becoming “incontestable” after five years. There’s a catch: the feds will not register a trademark if it “may disparage . . . persons, living or dead, institutions, beliefs, or national symbols, or bring them into contempt or disrepute.” (That’s the basis on which first the Patent and Trademark Office and then one federal judge have supported de-registering the Washington football team name.) In 2011, Simon Tam sought to register his all-Asian American dance band’s name — The Slants — in a bid to “reappropriate” that slur as something more positive and prideful. Officials denied Mr. Tam’s request as a violation of the rule against disparaging brands. He appealed to the federal court that specializes in intellectual property matters, which struck down the law against disparaging trademarks as a violation of free speech.

We agree with Mr. Tam, and so did most of the justices, judging by their remarks at oral argument. Still, the court should not sweep too broadly if it rules in Mr. Tam’s favor. The government’s lawyers have a point: Trademark registration determines a commercial brand’s eligibility for certain governmentally protected legal rights — everyone still has a right to take on any name he or she wants. The purpose of trademark registration is not cultural or political but commercial: to aid identification of various products and services in the marketplace. It’s reasonable to place conditions on registration, provided they are crafted carefully to prevent patently racist branding from skewing or blocking the channels of commerce.

The existing law is too vague — “may disparage” is subject to wildly varying but equally reasonable interpretations, as shown by Mr. Tam’s and the trademark office’s defensible but contrary readings of “The Slants.” Litigants presented evidence to the Supreme Court of the frequent inconsistency with which the “may disparage” standard has been applied. Though not quite “madness,” as a friend-of-the-court brief for the Washington NFL team claimed, the government cannot claim to be acting constitutionally if it’s not even acting coherently.

The justices should strike the “may disparage” clause without foreclosing the possibility that Congress might come up with a more precise and workable formulation. That’s what the First Amendment requires, even if it means Greater Washington must meanwhile rely on community pressure, and a stubborn owner’s sense of decency — not a lawsuit — to get rid of the team’s offensive name.