This sounds so outrageous. Don't people realize that just because someone has a developmental disability doesn't mean they lose a constitutional right. These folks have the capacity to learn -- it's a matter of finding their learning style and taking small steps.
In Minnesota, a dispute is going on about whether individuals with disabilities who are under the care of a guardian should retain the right to vote. According to the Star Tribune, Minnesotans whose affairs are controlled by guardianships do have the legal right to vote, unless a judge takes this away.