They don't just use Asperger's for that purpose. This is called an "ad hominem" argument and it's an invalid debate strategy that people use all the time.
I've seen things like, "What would YOU know about the economy? You voted for Bush!" Or, "You're only fifteen; you don't know anything about kids!" Rather than addressing the argument, they attack the person; and if they want to use this strategy they'll use just about anything they know about you to attack, even if it's completely irrelevant. I've even had my argument dismissed because I'm female... the argument in this case was about whether gardening was cheaper than buying produce at a farmer's market, which has nothing at all to do with gender.
Similarly, I've seen people go the other way--assuming I know everything about autism simply because I'm autistic. I have an advantage when it comes to knowing about autism, yes, but I can still be wrong, and have been wrong in the past, and probably have some ideas now that will eventually be disproved. Assuming that someone is an automatic authority because of who they are or what their background is can also be a fallacy.
Anyway, yeah, on the Internet sometimes people will assume Asperger's means you're either a whiny misfit looking for attention or else mentally ret*d (and treat you the way they treat mentally ret*d people, which tends to be totally unacceptable treatment at best). They will assume this because they think that anyone who discloses a disability is either whining about it instead of "overcoming" it or else is lying about it to get attention. Their basic assumption is that disability is intrinsically shameful; so you have to be a pretty pitiful person for admitting to having one.
But of course you can see where all this comes from--these people are often narcissists who feel threatened by the existence of people who can be open about their own shortcomings; so they use the usual strategy of attempting to make themselves look better by making other people look worse. Why do you think they would be so extreme in their abuse, if they didn't feel as though they themselves were being attacked in some way? Whether it's you that makes them feel threatened, or whether it's just the fact that they constantly feel the need to prove themselves superior, it's much more about them than it ever was about you. The best you can do when you encounter a place like that, where the majority of people are abusing you for being autistic, is to quietly leave, without announcing that you're leaving, and let them tear each other apart instead.