Probably are a few cases where the "assistant" is actually the one voting their own preference rather than the disabled person. But it's worth it to put up with a little of that rather than officially strip people of their voting rights on the basis of disability.
I've actually helped disabled people fill out voter registration for more-severely-disabled family members in the past, and while this sort of thing does happen, I don't think it's necessarily that out of line for a loving caregiving relationship anyway.
