If you're going to hold one party to the letter of the law, don't be surprised when the other party applies the letter of the law.
$15 an hour to cart bags around? That's more than the agents are making for essentially doing more work.
Mark, I'm not so sure it's enforceable. If I as a customer insist on giving a tip after being told you can't accept it, at some point you're now offending me. IIRC, even AA had a policy at one point which said you had to refuse a tip twice. If the customer still insisted, you could accept it without risking discipline.
It is enforceable. The employee contract is will be something along the lines of: "Don't take tips. If you take tips, you will be reprimanded, and possibly fired." Enforced "no tipping" policies exist in other businesses (i.e. some bus companies), this isn't any different. I believe that certain airlines (not sure if AA among them) have "no tipping" policies enforced at their lounges.
If a customer gets offended because an employee is following the rules and wishes to keep his job...well, then that's too bad, and it is extremely disrespectful of the customer to not respect an employee's regulations.