As always with these debates, it becomes one about the ads themselves rather than about the policy behind the decision. Then people add in a whole bunch of heated rhetoric about free speech, as if somehow the Constitution guarantees an ability to place advertisements in or on the sides of publically-owned buses (what if the City passed a no-advertising policy? would that be a violation of free speech?).
The question ought to be about the policy:
"The decision was made based on a subsection of the transit advertising policy which says that "religious advertising which promotes a specific ideology, ethic, point of view, policy or action, which in the opinion of the City might be deemed prejudicial to other religious groups or offensive to users of the transit system is not permitted. Religious advertising will be permitted if the information is designed to promote a specific meeting, gathering or event and the location, date and time of said event.""
Had the ad been about a atheist gathering, it would have permitted under this policy, just as ads for celebrating Christmas and other events are permitted (i.e. "Come celebrate Christmas by joining us for a performance of Handel's Messiah"). What isn't permitted are religious messages unrelated to an event, which, this ad is.
So do you agree with the policy or not? Should fundamentalist Christian groups be allowed to place "Repent or burn in Hell" type ads on buses? Should a Muslim group be allowed to place a message of the "There is only one god..." variety? One can hardly demand that atheist messages be permitted but religious messages not be.
What the Humanist group has failed to realize is that the religious groups have obeyed the existing policy, and they haven't. They aren't being discriminated against - they just didn't look at the policy. And they ought to know that in Ottawa of all places, we're of course going to have a policy

.