Freedom of whine

Ottawa atheists are crying censorship, the Citizen reports, because OC Transpo won't run their ads on buses. Must one point out once again that freedom of speech means not that you have a right to an audience, or a publisher, but only that if you find both the government will not forcibly intervene to silence you? If they claimed instead that they'd been denied equal treatment before the law (because public buses run some people's ads but not theirs) I'd be a bit more sympathetic. Only a bit, because governments quite properly tend to regulate public spaces not so everyone can say whatever they want but so everyone must avoid highly provocative or offensive displays in the street. But at least the atheists would not be putting forward a blatantly silly and petulant pseudo-constitutional argument which, among its other flaws, makes me doubt that their ads would impress me if I did see them.