The cheese more people choose...

I'm watching the George Carlin/Mark Twain special on PBS. Jon Stewart just said "cheese-tits" several times during his monologue, and PBS censored all but one of the utterances (the first). Why is the word appropriate once, but not two or three times? Is one "cheese-tits," by itself, not inflammatory, but multiple "cheese-tits" completely unacceptable?

I really don't care about the answer, I'm just happy I live in a nation where I can publicly exclaim "cheese-tits!"