I think our man Hotpoint could probably give you the proper reason as to what makes a fully auto paintball marker illegal, or even classed as a firearm. I seem to remember that it was something to do with muzzle energy or something, rather than how the projectile itself is fired from the "gun", in combination with how many projectiles are fired per single trigger pull.
I dunno...
Anyway, as to why we shouldn't have fully auto... I don't think muggings would be as much fun for the mugger, or the muggee for that matter, if you could just walk round a bunker, smooth as you like, firing around 20bps effortlessly. That would suck.
D.