I suppose it's context, ignorance and situational. Amongst gamers for example, especially in competitive communities, flaming is quite prevalent and members will use anything they can to get a rise out of a competitor or member they don't like.

Not defending it, just indicating that in most cases the users of such words have nothing against genuine cases of autism or whatever other condition that comes to their mind... it's just a means to an end to get the other person pissed off.

Some sites will differ in their rules on what is allowable in the context of flaming or trolling (two concepts that don't always go hand in hand) from very lax to very strict... so yeah, in some you're going to come across it.

Sometimes a useful site to ascertain why words are used in a demeaning way is the Urban Dictionary. In terms of 'Autistic' it has (unusually) a lot of informative information in this instance... but as you scroll down you'll see how it is also used in the context of an insult towards people who are not in fact autistic.