So a lot of people will explain it with many very interesting psychological and sociological theories on discrimination and shits
But to summarize it all :
they are
really
fucking
dumb
They can't understand two thirds of the shits happening so when faced with something new they simplify it by interpreting it as close to the closest thing they understand. Except they are fucking dumb so the closest thing they understand is miles away from the thing they are discovering.
Therefore when they encounter autism, they understand it like the closest thing they actually know, which is children or more likely stupid and they talk to children like they are dumb.
Now the other things that lead them to that is that despite being world class idiot their job is to evaluate people so considering they don't understand most things they require even more simplification.
TL;DR : tell them you know damn well they're not supposed to require an eval for autist so you're gonna sue them, should shut them the fuck up