However, I'm guessing you're actually talking about morality on an individual level -- that it's immoral for an individual parent not to try and guarantee their future child the happiest possible life. The problem is that even though being autistic is hard, having a hard life doesn't necessarily mean having a bad life. Being autistic is hard because autistic people are different, not because autism is inherently an undesirable trait.
Not everyone who is autistic or who has Downs would, if given the chance, flip a switch and erase that part of their personality, and they bristle at the "happy life" argument because they see it as (intentionally or not) just another way for people to imply, "differently abled people will always lead less satisfying lives." See also the controversy in the deaf community over hearing aids, which are a much less dicey proposition than eugenics but still prompt heated arguments sometimes.
As a side note, it's worth mentioning that no one is talking about artificially increasing the number of differently abled people as some kind of "recruitment strategy". Opponents to this kind of genetic selection are talking about just removing that characteristic as a determining factor entirely. It's not any different than banning sex-selective abortions, which is already an entirely normal and relatively uncontroversial policy in multiple developed countries.