My roommate and I experienced something like that a few months back (her car) and posted about it here:
http://community.livejournal.com/davis_square/689797.html

Definitely somewhat shady, esp. since at first the cop supported charging her the fee to not be towed, then supported lowering it to the amount of cash she had on hand, then when we put up a fight finally made a call and suddenly we could go to Pat's Tow to get her money back. Sketchy.