yes as said if clamp can be removed without damage to clamp then they can't do you for that but like said i too would want to know

1.) why i was clamped in first place
2.) if its rainway property do they have a clamping contract from the rainway company
3.) if they have the contract for clamping where there signs up on walls in and around area that could be seen as you parked

the clamper may just have like your car and decided that he was going to clamp with a view to removing it to pound and then trying to charge you excessive amount for release (to which you maybe couldn't afford )