They're called "bottom bracket dynamos" just because that gives the best indication of where they mount, as opposed to "sidewall dynamo." Both sidewall and BB dynamos are tire-powered, as opposed to "hub dynamo." You're right, of course, that it's not a very helpful name... but I can't think of a better one.

I had one for many years, and liked it because it didn't mess up the lines of the bike as bad as a sidewall dynamo. It seems very nicely made, and felt like a high quality unit in my hands. But in retrospect I must admit it wasn't actually a great dynamo. It was difficult (okay, impossible) to adjust the pressure of the spring, so on one bike there was too much pressure, on another there wasn't enough. If there wasn't enough, it would slip intermittently, especially in the rain when you most want it to be reliable. Of course you assume your wheel is round, but in practice it probably isn't; so you get flashes of light as you ride. As a result the dynamo spent most of its life in the parts bin. About five years ago I finally found a bike on which it fit perfectly (mounted to the kickstand bracket) and was happy to be using the dynamo... but the bearings conked out after about one mile of riding, and that was that.