I don't like it one bit, but from the government perspective it could make economic sense.
State Governments give tax breaks to businesses to spur the local economy all the time. In this case, the "business" is already a tax exempt, non profit entity. So, maybe the State can help that business attract better players by not taxing their income. Better players means better teams, which means happier fans spending more money!
Yeah, it's a stretch, and I don't even buy it, but it's not insane. The State tax is only about 5%, anyway.
If I was a professional athlete in those states, though, I would want to know why only college athletes are getting this gift.