Even if they can't prove the people who broke the seats are genuine Motherwell fans, the damage is caused to the Motherwell section, so the bill for any damages will go to Fir Park.
Also if the tickets corresponding to any broken seats were sold at Fir Park, the first thing Motherwell will probably do is look to see who bought those seats, which could mean innocent fans getting the blame for something they may not even have done, because a large number of people wouldn't even have sat on the seat their ticket was for.