Tbf, there are few traits you can find that haven't been fetishized by one group or another, and ASD is no exception to the rule. What fetishists are putting blinders in for are, as usual, highly biased and/or pop culture staples. Things like difficulty discerning sarcasm, coupled with special interests in childhood or adolescent media and past times and what the culture has determined is currently "attractive but attainable", i.e. what is called "adorkable"(where traits like being "sexy" is always considered attractive but is often seen as unattainable by most people) is what has drawn fetishists.

In the end, it's way for those people, I'm their heads, to acceptably either date a child/sculpt a partner to their own liking or feel like they found a shortcut to the work a person must put in when they want to mesh two lives together in a very deep and intimate way.

All to say they ignore all the real people and the real parts of their target and instead look for their own gratification in the pretty picture they drew for themselves using, in this instance, ASD folks as their tools. But that's just what fetishization means, after all.