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.
I was looking at it from the psychological perspective of someone who wanted to plant a bomb. I would think the terrorist wouldn't want anyone else to have access to the car, and would be very afraid of anyone finding out what he's up to. Just MHO though.
Right after they pull up your GPS and get the address for your now vacant home.