This is actually false (for most of Cambridge, at least). Sidewalk riding IS legal in most places (major pedestrian centers like Harvard Sq and Central Sq are off limits and signed as such), however bicyclists are limited in speed to that of the slowest pedestrians, and must always yield right of way to pedestrians. There's really nowhere in Cambridge that I would consider riding the sidewalks a good idea (except when avoiding some road construction), but it is wrong to say it is illegal.

As for the OP's claims... if you've spent any time in South Florida his/her viewpoint would make more sense. There, roads between developments are primarily 4 to 6 lane boulevards that only the bravest of souls would dare try to ride on, and drivers regularly travel at 60mph. As a fairly strong supporter of VC, even I would think twice about getting on a bike and riding like I do on my commute through Boston (or other, smaller northeast and west coast cities I have ridden in). Also, sidewalks there may as well be bike lanes as they are often wide, relatively un-obstructed, and since everyone lives in a gated community, driveways are pretty much non-existent.

The problem that the OP has observed is not that bicycling on the roads is inherently dangerous, it is that the roads in South Florida are just plain dangerous (for all users). Given some proper urban planning and proper enforcement of traffic laws, it could be perfectly safe to ride (and these things would be a MUCH better solution than building an elevated bikeway, IMHO).