Yes, the problem is that USB can reach very high speeds but only in bursts. Firewire is not always as fast, but more constant. Over time they average out to about the same speed. When you’re talking about copying something from a hard drive or an iPod it should take about the same amount of time, and so either is fine for things like that. But when it comes to video you can’t afford to have a drop in speed for even a moment because you’ll drop frames. Thus, most video cameras and video programs are designed to use firewire. If your camera has USB on it your guess about copying photos is probably correct…it’s most likely what that’s for.
You may see a consumer camera every now and then that copies video by USB but it is not as reliable and should be avoided if at all possible.