I wouldn't say that fighting bettas (at least the fights I have read about) as cruel as dog fights. The bettas used to fight are quite frankly pampered, and the fights only last until there is a clear winner... dog fighters starve, beat, and leave their dog's wounds untreated. The dogs fight to the death also.
Male bettas involved in fights are treated for their wounds immediatly after the fight is over, whether or not the fish needs it. Male bettas, it seems, are treated more like a boxer (not the dog breed) than a fighting dog.
Don't get me wrong, I don't agree with fighting male bettas. I just don't agree with the previous statement either.