Though I'm sure there are exceptions, the usual rule seems to be that Mac applications that do this are applications that have only one window, so that there's no loss in identifying the application itself with this window. As far as I can tell, you can only have one GarageBand project open at once, so it falls into that category. I think the one-open-project limitation is kind of a pity, since it prevents you from, say, dragging tracks easily from one project to another. But iPhoto works that way too, and I think iMovie does.