It's only my opinion, but I believe the TSA wants to do a reverse screening at the destination airport to keep prohibited items from advancing into an aiport's sterile area. Having the police escort the passengers out of the terminal would most likely not endanger anyone, but the TSA wants total control of unscreened items entering the sterile area.
<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Originally posted by whirledtraveler:
...Why can't the police follow you out of the airport and stop you on the street?
</font>