I think that parents of autistic children know what their children can and can not handle. That is why they ask for DAS. I think that many, probably even Disney, don't fully comprehend how these medical issues manifest. It is easy for someone to say, "well, if your child can't handle the line anymore, then just leave for a while". But autistic children/adults are not like typical people. Many times they can get too worked up and it can take a long time for them to calm down. Some even have to leave the parks for the day.
So just because the leaving the queue accommodation seems reasonable to you, doesn't mean that it is.
And how in the world can Disney possibly know which autistic person is capable of waiting in line and which are not? We all know that their decision is completely arbitrary. If they are looking for specific phrases, once those phrases get out, then everyone is going to use them to get a DAS.