Because the issues with many people inflicted with this is they can have triggers around patterns and wants.. many can not handle 'change' very well. So their schedule and patterns are very key. Unlike a physical handicap where you are simply trying to avoid a physical constraint... in many of these cases you are dealing with mental and psychological issues that you can't simply rationalize with like you could another person.
And that's really where the law needs to start making the distinction, and hopefully the current lawsuits will lead to where the responsibility lies.
That's going to be a challenge, because it's so subjective in many ways, and honestly, a lot of it in many cases depends on the attitude of the parents towards their child's disability.
There is one camp that simply believes that they have no responsibility for helping their child to cope as best they can in the world, and that the world just needs to totally shift around their needs/wants. Anything that requires their child to adapt, is discriminatory or offensive.
Then you have other parents who try to equip their kid as best at they are able to function in the world as possible. For example, I have known parents that "practiced" standing in line at home well before a trip to some place like WDW. Starting just around the house. And then working up to some place public. Working with them to develop strategies and to find out what works for them.
Now, obviously, that doesn't work for everyone - but in many cases, it can - unless there is a truly severe disability. And it only makes sense - you have a kid that hates or can't wait in lines - but if you never work with them to wait in one, and they are just used to walking up to the front of the lunch line at school every day, and never being exposed to them - clearly, they never have the opportunity to even possibly overcome or be in any control of it.
It's a sticky situation, no doubt - but clearly I think it is one that some sort of determination has to be made legally.