In fact, ADA rules allow airlines to do this.
Carriers may require the following individuals to be accompanied by an attendant....A person with a mobility impairment so severe that the individual is unable to assist in his or her own evacuation from the aircraft.
The carrier and the passenger may disagree about the applicability of one of these criteria. In such cases, the airline can require the passenger to travel with an attendant, contrary to the passenger’s assurances that he or she can travel alone. However, the carrier cannot charge for the transportation of the attendant.
I find this unfair and discriminatory. A Wheelchair user is then left to any airline employee's opinion about whether they can fly that day or not. How can a wheelchair user ever make a flight alone?