CADad, I wouldn't say it's necessarily an adaptation for protection. It seems to be more a matter of not taking care of the biceps enough. Normally what a contracture like you describe is simply due to imflexibility of the biceps brachii.
I think, to an extent, most baseball players will experience this. I recently noticed that I have just a little bit of one. I've noticed it while lifting weights that my throwing arm does not appear to straighten out all the way. It's not terrible, but just a little bit.