Star Trek 3's Excelsior had them too.I don't know, I've seen them thrown from their seats pretty damn often, especially on TOS and TNG.That seat belt was stupid. And way too self aware. There's no reasons to have seatbealts on a starship. Collisions that knock them out of the seat are the rarest exception ever.
The Motion Picture had already introduced these to keep you in your seat in case it gets a little rough:
http://movies.trekcore.com/gallery/albums/tmphd/tmphd0812.jpg
http://movies.trekcore.com/gallery/albums/tmphd/tmphd0830.jpg
Makes a lot more sense to me and isn't as silly as a seatbelt.