Because we have a FREE PRESS. Newspapers are not required to publish anything they PERSONALLY find offensive. But I have the right to start my own newspaper and print just about anything I want (short of Child Pornography) and nobody can sue me for anything I say that isn't actually maliciously libelous. Apparently in Canada if you offend the sensibilities of "queers" by calling them something like... well, "queers", you can be sucessfully sued and possibly arrested and prosecuted.
The difference is whether or not the government gets involved in the process by prohibiting the behavior, or allowing frivilous lawsuits like this one to go forward and then upholding it on appeal.
So , given the advertisement in question, it would , as you say , be difficult to find a publisher who would not be offended and publish it. Knowing he has free press and would not be sued or charged. But declining simply based on his own moral judgement and sensibilities?
Interesting.