You Know It's Not Polite, Right?

I had previously assumed strangers were more likely to ask me why I have a service dog than they would be to ask someone why they have a wheelchair, but perhaps I've been mistaken about that.

Yesterday I was talking to one of my neighbors, who wears leg braces and walks with a cane.  She told me that strangers will come up to her in the grocery store or another public place and ask her why she wears braces or what's wrong with her.  Sometimes they ask, "Did you have polio?  Do you have cerebral palsy?"

You know that's not polite, right?  Please tell me you don't do that.

I assume most adults know that's bad manners, but many just choose to do it anyway.  I don't understand why, though.  I mean, I get being curious.  But I don't ask complete strangers every little thing I feel curious about.  I know it's bad manners to ask people I don't know personal questions and I know it's none of my business.  So I keep my curiosity to myself.  Why is it that some people think they have the right to information that's really none of their business?