Folks who make the confusion do so from reading books and magazine articles about autism, and not from encountering actual autistics/aspies.
And the reason is that they get hung up on the "lack of empathy" thing, and get "empathy" confused with "sympathy".
Empathy is theory of mind. Sympathy is concern/caring.
ASD folks have caring, but don't have theory of mind. Socipaths/psychopaths (basically the same thing) are the exact opposite, and survive by exploiting their accurate TOM, but have no concern for their victims.
NTs who actually encounter actual ASD folks could never confuse an autistic with a psychopath.