Because they can and its a free country.  I don't know.  Maybe it has something to do with the fact nobody wants to sign an RB with a "weed cloud" hanging over his head who if and thats a big if, decide to take a chance on, won't want to pay him big bucks just so that the  league can suspend him again for violation of the NFL's substance abuse policy.  Other than that I really don't know what the concern would be.