You speak as if this is unusual. Randy Couture, B.J.Penn, Kenny Florian, Rashad Evans, Vitor Belfort, Lyoto Machida, Diego Sanchez come immediately to mind as doing this and I could go on and on. Many times its a step up in competition when a fighter steps down in weight class. A pertinent fact that you seem to be overlooking is that the fighter himself has to shrink his body to fight with the smaller guys, and rest assured, there are plenty of guys waiting for him who themselves have shrunk their bodies from a naturally larger size as well. So, to answer your question, no, it does not seem like a bitch move.