JMO, I think it's pretty pointless to argue about whether autism etc. are mental illnesses or neurological. The whole distinction between mental disorders and neurological/other physical diseases is a remnant of an older age of thinking when it was thought that there were diseases of the body and diseases of the mind where they couldn't find any underlying body pathology. But the more we learn about diseases and disorders the more we find that there are physiological factors in mental illness and mental factors in neurological illness. The line is going to become more and more blurred imo.
http://bjp.rcpsych.org/content/178/6/490.full
Autism is NOT a mental illness. It is a developmental difference that occurrs in some people. It is no different from being very tall or very short.
There is a whole graduation in the population from severe (the equivalent of being a dwarf for example) all the way to the center of human behaviour.
There is no clear cut off and there is no organic basis for it. So it is not a disease.
What does happen however is that because these people behave differently from most, they tend to be come socially isolated and that is what actually causes the anti-social behaviour sometimes seen.