I think it's because they think that the mental illness may be demon related. Multiple personality disorder--this has a new name in my psychological book, but this is the name I remember--may be compared in their minds to the man named Legion in the Bible. Some people might think that the person sinned to cause it--there are a lot of people who think God always blesses those who do right--but take a look at the book of Job and you will know that is not the case.
I personally am shamed by people's inconsiderate nature toward mental illness. Some people actually act like they are mentally handicapped--which isn't actually the case unless its severe. It's just horrible to me because some people have gotten to the depressed state they are in because they feel like no one cares and then when no one does, they are even more likely to sink into self-destructive behavior.
Now people even claiming to see spirits, demons, or angels can be told they are mentally ill--even by fellow Christians. It seems as though they have forgotten all the people in the Bible that did just that and they weren't considered crazy.
I guess the ultimate solution to this is the modern world's attitudes and the idea everyone must be the same way in certain instances. I say to these people, just leave people alone and act like you care. Unless these people are a danger to you personally, forget about it and move on if you can't feel sorry for them.
The blind part is more of a physical ailment and is not intolerable behavior, so they are more moved to compassion. Blind people generally don't act out in unusual ways, so they find it easier to assist them.