Yes you can. Its called the Americans with Disabilites Act and if it is an "unfair" advantage, why isn't he out there winning tournaments? An advantage gives someone a means to have a leg up on everyone else. It isn't an advantage if you are using it just to have a fair shot. I fail to see how using a golf cart helps your golf game anyway. It doesn't help his golf game, it helps him survive and any reasonable person would know that it isn't an advantage.
Golf cart an advantage? Not in my book! Should carts be permitted under special circumstances....YES! Players with a disability should be encouraged to pursue (or continue to pursue) professional golf. It's not how YOU get to the hole......it's how the BALL gets there. Players like Casey Martin and Eric Compton should be permitted use of a golf cart IF they feel it is necessary. The Champions Tour permits them assuming that all pros over 50 have the need solely due to their age.