Such parents are emotionally overwhelmed and seek sympathy, believing the Internet to be a harmless outlet for venting (and indeed, it's better than complaining about a child's behavior in front of the child). They don't stop to consider the cumulative impact on society—the tremendous amplification of prejudice—when large numbers of parents are talking only about the negative aspects of raising an autistic child.
Zilari also notes that such parents, even if they initially seem hostile to the notion of acceptance, often are willing to engage in productive discussion when they discover that there are many positive and constructive views out there. This is indeed a very good thing.
In other contexts, though, whining and productive discussion aren't necessarily opposites. Not all complaining is unproductive, not all discussion is useful, and not all ideas deserve a thorough hearing. Not Mercury just posted a hilarious illustration of why certain kinds of harmful ideas need to be summarily junked. When we're talking about quack-cure fraud, or eugenics, or child abuse, we can do a lot more good for society by screaming about it at the top of our lungs, rather than having nice quiet civil discussions with the perpetrators. Evil thrives in the dark. Sometimes we need to put on our steel-toed boots, kick over a few rocks, and stomp real hard on whatever crawls out from under them.
As I've said before, I have only four words of "discussion" for Autism Speaks and Cure Autism Now and any other sicko bigots who think that a global genocide would be an act of charity: Kiss my autistic ass.
And what's more, I have a very specific goal when I'm writing about their repulsive ideology, which is to make their research and their other activities so controversial—so totally radioactive—that when they send begging letters to corporate donors, the recipients will pick up the letter with protective gloves and circular-file it in the nearest hazardous waste bin.
Sometimes you just gotta do some whining.