You do realize you are talking to an autistic person right? Just making sure you remember that.
Anyways if they are capable of it, living on SSI or not, it's good for them to get some sort of independence even if it's living with others in a similar position. I'm sure they more than likely are monitored by counselors in some sort of program or might even live in an apartment complex geared towards getting people with disabilities that are capable of living on their own, to an extent, out of their families house. There are a few of those type of places around Seattle.