Because it signals "I am so wealthy that I can afford to tear up money" and blatantly signaling wealth is crass. And it also signals "I am so callous that I would rather tear up money than give it to the poor", which is also crass. And the argument that a one dollar bill really isn't very much money isn't enough to disrupt the signal.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.