sidewalk biking

I've just moved to Ann Arbor. I used to live in Boston.

There are plenty of bicycles here. Not as many as there should be, but a start. But they've started with a problem: the vast majority of Ann Arborites (or is it Ann Arboreans? What are the rules for this sort of thing?) ride their bicycles on the sidewalk. You are walking along a sidewalk and lo and behold you find someone cruising down the side of the walk you are using. And I don't understand it. Why are so many people riding their bikes on the sidewalk? My gal sal says that at least folks are riding bikes, and she highlights the fact that so many people are scared of riding in the streets. Moral: we should be thankful for the sidewalk riders, for they are riding bikes and not gas-guzzling. I see her point. But Ann Arbor isn't exactly Grand Central Station, especially in summer. My question: How does this sort of thing get started? How does an entire town come to think that it is okay to ride bicycles on the sidewalk?