The airline I currently work for is the only airline I have worked for, so it has never seemed that strange to me. The company says that since bringing in this procedure, the amount of money the company makes has gone up!! (Feel free to correct me if anyone knows this comment to be purely galley fm lol!!)
I have never really considered the legality's of this issue, as I assume it must be ok as it must have gone through the union etc when it was brought in...
They also have the rights to randomly search us after a flight...and when I say search, I mean search every little last nook and cranny of your crewbag, handbag and you are asked to remove your work jacket, coat and shoes!! LOL
To the person who mentioned flight attendants in the US aren't allowed to accept tips, this really shocks me as like you mentioned I thought you had to tip EVERYONE in the US!! lol And if you are already paid for the service you are offering, then so are taxi drivers, waiters, porters etc.