Ronald,
My understanding is that they can refuse a character that does unnecessary damage, and have the right to restrict access in inclement weather, i.e., if they don't mow the greens (or wouldn't) due to wetness, then they don't have to allow other vehicles out there.
The problem as has been pointed out, is that the law is vague, and deliberately intentioned to be fleshed out to further conclusion via complaints, further rulings, and lawsuits. That is always scary to golf course operators, and the counsel most get is to be as accomodating as possible to all ADA golfers (even borderline cases) in an effort to establish a relationship with them. I suspect one of those test lawsuits will come against a signature architect or famous owner (Trump maybe) or well known resort or club for the ADA activist to get the most publicity out of it to state their case.
As an aside, I have been designing greens, tees and fw for 15 years to ADA rules as they were/are in place. However, it surprises me how many architects, contractors, owners, etc. are totally unaware of the rules, or need to conform to them. For the most part, making the green easily accessible isn't much different than what I would do to make it easily walkable from the cart path to a senior golfer, of which I am becoming rapidly.......and can feel their pain!