Because fat people are denied jobs, housing, and respect...tall people are not. As far as the bear heads...that's hilarious...but not at ALL comparable to demeaning minorities. Same for the cat thing. They don't promote the mistreatment of particular groups.
"I also think that there are so many offensive door mats, t-shirts, bumper stickers, door signs, tattoos, jewelry, posters and conversations, that you can't confront them all, so you have to pick your battles, but if you only choose to fight those that are aimed at a group you belong to, then you're a bigot in your own right (I don't care who you insult as long as it's not me and my group)."
That just proves my point that we need more people to speak up. I certainly don't have time or energy to chase down every offensive bumper sticker I see...but if it's something I see on a regular basis or is right in front of me. I have an opportunity (and a moral obligation) to speak up. And how does it make someone a bigot if they reserve most of their effort to defend their own disadvantaged group? Was MLK a bigot? César Chávez? Harvey Milk? They devoted the majority of their
lives to speaking out against hate aimed at "their" groups.
You might not be offended by it Kaplods, but a little kid might be hurt by it. Hence...moral obligation.
Yes, it serves as his own punishment. Karma is a...wonderful lady.

That doesn't negate the fact that someone should say something...actually it is kind of like saying..oh well, someone will eventually set him straight by not banging him so I don't need to say anything. The bystander effect, as we know...doesn't help anything but our individual convenience and comfort.
I would also suggest that there is a BIG difference between giving him the "rise" he may or may not be seeking and calmly offering your opinion and then dropping the issue. For instance, while I literally lol'd at some of the suggestions offered here (no fat chicks allowed, just dicks, etc.), I would consider them "rises" as opposed to calm, informative action.
Again, apathy never accomplished anything but personal comfort.
Obviously we have to choose out battles...but this one happens to be something the OP has to see regularly, in her neighborhood...which reasonably calls for action.