Seems pretty silly to me. Anyone who is going to willingly hand their keys to a stranger to park their car and leave the keys with the stranger, isn’t likely to have an explosive planted in the car that they are so willingly giving a stranger full access to.
actually, I'd think that would make it easier.
Figure that the valet isn't going to look too hard in the trunk, etc. They'll just park it and hurry back for the next pick-up.
That being said, I think that the TSA is waaaaay out of line here.