Sheltering straphangers from ... controversy?

Once again, the city that spawned the expression "banned in Boston" is facing charges of government censorship.

This time, it's not controversial plays or books being axed, but certain advertisements submitted to the city's subways and buses - involving the relative criminality of marijuana use and the "Christianliness" of Santa Claus.

Despite repeated losses over the years to advertisers suing on free-speech grounds, the Massachusetts Bay Transportation Authority (MBTA) finds itself facing two fresh legal challenges - both claiming that the public agency is overstepping its authority to regulate ads.

Disputes over the use of public advertising space - and whether free speech protections extend to advertisers - are common. But the frequency with which the MBTA gets tangled in these disputes - five times in the past 30 years - is unusual, especially given the Massachussetts liberal reputation.

"You read the transit free-speech cases, and it's like a travel directory of the United States," says Harvey Schwartz, the attorney representing both of the current plaintiffs. "But you don't see any repeat cases.... Boston is stubborn."

Indeed, many Bostonians say there's a more profound cultural reality at play: A deep-seeded Puritan morality and the strong political influence of the Roman Catholic Church that form a conservative alterego to the region's otherwise liberal public face.

One of the two new cases is a federal lawsuit - tried earlier this month and awaiting the judge's verdict - that was brought by a drug-policy reform group, Change the Climate. The Massachusetts group encourages debate about the fairness of marijuana laws. The MBTA argues that the group's ads tacitly encourage illegal activity.

ONE of the group's three ads submitted to the MBTA depicted a teenage girl, with the text: "Smoking pot is not cool, but we're not stupid ya know. Marijuana is NOT cocaine or heroin. Tell us the truth...."

In rejecting the ads, the MBTA says it was simply acting within its guidelines that prohibit ads that contain violent criminal content, are harmful to children, or denigrate groups based on gender, race, ethnicity, religion, or political affiliation.

"We are an agency that works with the public," says MBTA spokeswoman Lydia Rivera. "We owe the community a transit that they shouldn't feel uncomfortable with.... Other transits might agree [with us], but bend a little. And we don't. We refuse."

Indeed, Washington, D.C., for example, already ran similar ads from Change the Climate. Ever since its transit agency tried unsuccessfully in 1984 to ban an ad that ridiculed Ronald Reagan as president of the "jellybean republic," about the only major guideline it's had is that ads can't carry inaccurate information, says Lisa Farbstein, Washington Metropolitan Area Transit Authority spokeswoman.

Legally, a public agency like the MBTA is allowed to create certain rules - like banning pornography and profanity, for instance - so long as they are enforced equally, across political views. In the case of Change the Climate, the MBTA says the ads fall into the "harmful to children" category by tacitly encouraging illegal activity.

In the case of the Church with the Good News - the plaintiff in the second new lawsuit - the MBTA claimed the church's ad violated its prohibition against denigrating a group based on religion. The church's ad promoted its website which, the MBTA says, declares other religions false.

But the court cases contend that the agency guidelines are too vague. In the last similar legal battle, a suit brought in 1994 by the AIDS Action Committee of Massachusetts, a federal court handed down an opinion saying the MBTA used a policy so vague as to be incomprehensible, says Sarah Wunsch, an attorney for the ACLU of Massachusetts.

"You'd think they'd learn," says Ms. Wunsch, noting that in court earlier this month, the MBTA was still arguing that "Boston is different: 'We have standards here.' "

Indeed, those standards are the root of a recurring trend. From its Puritan roots to the notorious Watch and Ward Society in the 1920s which outlawed, among other things, Theodore Dreiser's "An American Tragedy" and Walt Whitman's "Leaves of Grass," Boston has a very long history of enforcing morality, says Stephen Nonack, head of reader services at the Boston Athenaeum, an independent library that specializes in local history. "The term 'banned in Boston' has got a familiar ring all over the country," he adds.