The reason doesn't even matter unless they are dumb enough to state something illegal as their reasoning. They don't have to give a reason at all.
http://www.afscme.org/blog/right-to-works-racist-roots
Anyway, I think we can all agree the NFL's handling of this, Hardy, Rice, Marijuana, and just generally everything related has been an outright disaster from the word go. It all speaks of arrogance, favoritism, greed, and stupidity that stems from too many rich suits and too many lawyers thinking purely of themselves with no regard for fairness, logic, or transparency.