1. Parental Blame and Guilt
Over and over, proposed causes blame mothers for somehow causing their child's autism. My own mom struggled with guilt for a long time over whether something she did during her pregnancy caused my sister's Asperger's, a thought which plagues her even now that my sister's an adult.*
Unfortunately, my mother also was one of many to have had the misfortune of experiencing the explicit blame for their child's autism very early on. When my sister was first diagnosed, the psychologist flat out told my mother that her lack of maternal warmth made my sister autistic. (Even ignoring the fact that this cause was debunked, I'd like to add here that my mother is one of the most loving and affectionate women I know.) My mother is the perfect example of the harm to parents that falsely proposed causes and blame can inflict.
2. Making Money off the Backs of Parents
It really bothers me when everything under the sun is supposedly linked to autism, and miracle 'cures' are offered as a way to fix autism overnight. I see it as snake oil peddlers taking advantage of parents with children on the spectrum. "Want to cure your child's autism?" they'll ask you. "Drink/eat/use this!" They wheedle with stories of other parents who've seen success, and it's pretty broadly implied that you're a 'bad parent' if you don't try (and therefore buy) anything and everything for your child. There is no cure for autism right now, but you wouldn't know it if you looked at the sort of sales going on online.
3. Publicity Seekers
Some people don't do it for the money, so much as the attention to their cause. After all, it's big news whenever something's linked to autism (free publicity!), and people don't seem to look too far checking to see if there is scientific (peer reviewed) evidence for different causes, so it's pretty easy to get away with linking autism to almost anything. So if a person thinks that X is bad and they want publicity for their cause, they might link it to autism in newsletters, websites, books (which also make loads of cash), etc.
4. First, Do No Harm
And speculating about the cause of autism without some serious evidence (I'd like at least two peer reviewed studies before I even started to speculate) can do a lot of harm, because supposed causes lead to actions, and actions based on false evidence can have bad result. There are consequences to every action, and so far all I've seen come of these speculations (on net) is pain. Children have even died because of this. That's not worth any amount of placebo effect.
I am not at all saying or trying to imply here that we shouldn't ever talk about the cause of autism, or try to find it. What I am saying is that we should do so carefully. So the next time you see or hear something about the cause of autism, proceed with caution.
*She shouldn't worry - I told her, as my sister would, that even in the very unlikely event that something she did during her pregnancy contributed somehow (and I really stress how unlikely that would be - there are a lot of studies examining correlations, but you really can't ethically do any causal studies, and I don't know how many of those correlational studies are peer reviewed), my sister would ask her to do it all over again. My sister likes who she is, and has made it clear she wouldn't want to be any other way.