i would agree with you if it had been the champion demanding it, but it wasnt it was the challenger.

cotto signed the contract with this clause cos it made him a lot of money, or he got some other concession in return (ring size, or glove weight or something) a gamble that would have been well worth it, if it had come off. but this is something that will always happen, as boxing is business. Unlike ufc where they are centrally contracted. correct me if i am worng but dont think there have been any catchweight title fights in ufc

but fighters dont always weigh in at the limit. eg hatton - pacquiao

138/140

and this is not even taking into account the weight they put on in the last 24 hrs.