I think you're forgetting that the majority of women who birth and keep their children love them unconditionally. There are people on death row for murder and rape that have mothers that still love the person who wasn't the murderer/rapist. Her situation with Adam is identical to so many other mothers of autists. The enabling, the constant cleaning up of their messes, it's textbook. Just look at Christory. Barb was exactly the same.
Point is, I'm sure she knows she's created the situation she's in, but had no other idea what to do. When you get that diagnosis, you start imagining the worst case scenarios for where your child will end up. So you dedicate yourself to protecting them instead of forcing them outside their comfort zone. Or, because autistic males have high propensity for violence, she became unable to push him to normalcy once he hit puberty and got bigger and stronger than her.
Again, this is all textbook- read any book on autistic adults or any scientific study. People fall into one of three categories once they get that diagnosis for their kid. They either A- become the enabler like Beth here, B- kill the kid because they convince themselves their life will be suffering especially after they die and the kid is alone, or C- put in the effort to understand the limits of their kid's cognitive function and help them to adulthood and independence with a lot of blood, sweat, and tears. Type C, sadly, is pretty damn rare.