The clamp might have corroded on to the rails on that saddle as there is no reason for it to be stuck like that.....unless....the previous owner somehow glued it on with epoxy or some other permanent type glue.
People have been know to glue stuff together if they cannot get them tight enough with their tools to stay in position. Do you see any sign of glue or corrosion between the seat rails and clamps??
Some parts also tend to mechanically hook and stick together after they are tightened on with bolts or screws either from parts "keying" tightly togehter at common openings or deformation. Give the clamp assembly a few taps or firm whacks with a small hammer or any other heavyish tool you may have handy. If your'e lucky, it might just ping off the rails and fall to the floor in separate parts.
Good Luck!

Chombi