Back when I worked at Six Flags, the requirement was that all riders MUST have three limbs. One of the three limbs coul be be a prosthetic limb, but had to have three. I don't remember, though, if they had to have at least one biological leg (it has been over ten years)...but there were rules.
In any event, the family has every right to sue, should they pursue that path. Despite the fact that he ignored warning signs, it was the park's negligence from preventing him to ride that caused the accident. The average person may not know WHY the rule is in place, and think it will be "ok." I'm sure even at Disney people try to "con" their way onto rides they can't ride because of some factor (too short, etc.).