How do they know if its your ID without employee interference?
How does a bartender know its actually your I.D.? These are driver licenses we are talking about here, by the way. In the U.S., fake I.D.s that are scannable are much more expensive than normal fake I.D.s (though, they do exist of course). I'm basing that information on no particular source, so feel free to refute/back me up.
Walmarts in certain states scan your I.D. when you try to by alcohol. It makes sense. You could still have someone over 21 over seeing the whole operation, of course. Cameras in the store also make it less likely that people will give drinks to minors (while in the store). If some guy loans a minor his I.D. so that he can buy beer, well...
This seems unlikely for a few reasons. For one thing, it means the actual owner won't be able to buy alcohol, since the machine limits consumption. Also, he risks having the card confiscated (the adult present could conduct random searches). Also, he would be without his actual driver's license for a while. If he just handed off the card while he and the minor were both in the store, he would be caught on camera.
All in all this system seems, if anything, more reliable than the system normally used in places that sell alcohol.