I think as others have said it’s one of those things that lots of people are ignorant about, if they have no direct experience themselves, but it’s also one of those things where actually sometimes even having some experience can make it worse.
ND is such a broad spectrum and 5 people with exactly the same diagnosis can all be affected so differently that it can be really hard to understand. So yes some people are ignorant of it and that’s where their perspective comes from, but I do think there are also lots of people who have SOME experience and that actually then fuels the nastiness on here.
As an example my nephew is autistic, he is quiet & struggles with eye contact/communication, struggles with loud noises and busy environments, lines his toys up, has to know exactly what he is doing at exactly what time and why, every single day, LOVES school & learning, but if you saw him playing in the park or walking in the street you would never guess he was autistic, and if the only autistic person a person knew was my nephew then they use that as their mental picture of what autism looks like.
On the other hand, my BIL’s nephew has autism, so same diagnosis, but it impacts his life very differently. He refuses to get dressed because the feel of the fabric on his skin is overstimulating so he is often wearing PJ’s, he can only eat specific foods and if offered something else he will scream and throw things as well as physically be sick, he can’t cope with school- it is entirely too overstimulating and he was having repeated meltdowns in school to the point he now is only there for 2 hours per day awaiting a place in a specialist school.
You have people who only know someone like my nephew seeing posts on here about someone like my BIL’s nephew and commenting that the behaviour has to be due to parenting because “I know someone with autism and they don’t behave that way”.