Are you referring to the professional level of the
sport ? Since there are different organizations which promote the sport of MMA it may be a decision made at the front office of each . Its also possible that it is regulated differently from state to state as with professional boxing, etc. The major sports like football, basketball, baseball, etc. do not ban players with HIV,AIDS,or HCV.
Perhaps inquiring with one of the org's that promote the sport could get you an answer.