The Massachusetts Bay Transportation Authority does not agree with the messages that Change the Climate, a drug policy reform group, wants to put on trains and buses in Boston. But it can't just say that.

As a government agency, forbidden by the First Amendment to pick and choose among viewpoints, the MBTA has to pretend it has other reasons for turning down the ads. In a trial that begins today, a federal judge will decide whether any of these fig leaves is big enough to cover the transit authority's naked attempt to squelch dissent.

The MBTA has had difficulty articulating a rationale for rejecting Change the Climate's ads. "Change the Climate promotes the use of marijuana in a suttle [sic] way and also is really a reform marijuana [sic] in a [sic] effort to legalize," wrote Lucy Shorter, the MBTA's director of marketing and (believe it or not) communication, in a January 2000 note that was supposed to explain the authority's decision.

Contrary to Shorter, whose reading skills seem to be on par with her writing ability, Change the Climate's ads (which have already appeared in subway stations and on buses in Washington, D.C.) do not advocate marijuana use or legalization. Rather, they criticize current marijuana policies as excessively punitive and wasteful.

One ad features a mother who declares that "jail is a lot more dangerous than smoking pot." Another shows a teenager who says, "Smoking pot is not cool, but we're not stupid, ya know. Marijuana is NOT cocaine or heroin." The third, which includes a picture of two police officers, reads, "Police are too important...to waste on arresting people for marijuana when real criminals are on the loose."

It's possible to agree with all these statements and still oppose both pot smoking and legalization. More important, even if the messages went as far as Shorter thought they did, they would still be protected by the First Amendment.

The Supreme Court has made it clear that speech may not be banned simply because it advocates defiance or repeal of existing laws. The Court also has taken a dim view of government attempts to exclude speakers from public forums because of their opinions.

In a clumsy attempt get around this obstacle, Shorter asserted that Change the Climate's "mission" conflicted with the MBTA's requirement that ads be of "reputable character"--a standard so vague that it gives transit officials carte blanche to reject messages that offend them. Bizarrely, she also claimed the ads violated the authority's drug-free workplace policy, which prohibits using or distributing drugs on MBTA property and working under the influence. The policy says nothing about ads.

The MBTA later distanced itself from Shorter's explanation, but its new excuses were no more persuasive. MBTA General Manager Robert H. Prince Jr. claimed the ads ran afoul of a 1995 policy against material "of a nature to frighten children...showing them language or pictures that describe or depict violent criminal or brutal activities...in a manner which causes a child physical or emotional distress."

Prince's reasoning: "This [smoking pot] could put a child in jail. That's emotional distress."

Lately the MBTA has decided the real problem with the ads is that they violate guidelines adopted six months after they were rejected. In particular, the authority says, the ads break rules against "violent criminal content" and "promotional materials that is [sic] harmful to juveniles."

With the possible exception of Robert Prince, no one who looks at these ads sees violence of any kind. And if referring to illegal activity constitutes "criminal content," the MBTA will have to start turning down all kinds of messages, ranging from ads for burglar alarms to the federal government's warnings about drugs.

But what about the children? "Why should a government entity be forced to put up a message that may be harmful to children?" Paul Cellucci, then governor of Massachusetts, demanded after hearing about Change the Climate's lawsuit. "That's ridiculous."

No, what's ridiculous is the assumption that a bunch of bureaucrats can objectively identify which messages are so "harmful to children" that they can't be tolerated. Such an exercise would quickly become a pretext for shutting out controversial viewpoints.

The MBTA has been repeatedly chastised by federal courts for doing just that in cases involving abortion, "safe sex," and animal rights. If law and logic prevail, it's in for yet another rebuke. Maybe this time it will get the message.