and so they should! clamping only means trapping the vehicle there, mostly in a illegal parking space, for an extended time potentially affecting the flow of traffic.
then there is the making money side of things if someone has illegally parked somewhere i think it would be fair to slap tickets on the windscreen and repeat if needed but clamping doesnt solve anything!
my mate had his car clamped parking in a residential space in london, he couldnt see the sign at the very start of the road and there were no lines. he waited 2 hours for someone to arrive to unclamp at a cost of £200 plus £60 for the ticket. absolutely ridiculous!