Of course there's a difference between shyness and social anxiety.
There's also a difference between shyness and autistic spectrum disorders, or depression, or hearing difficulties, or sight problems, or any and all of the multiple reasons someone may struggle in a social situation.
As I said upthread, I have a couple of friends who struggle and know they struggle (and FWIW, one has severe chronic depression and the other has Asperger's). And I have depression and sometimes I don't want to get out of bed, let alone make polite conversation. A lovely friend of mine is currently struggling so much with stress and depression it is actually affecting her speech. She's awesome and I feel rotten for her.
So I really object to the idea that anyone who thinks shyness is used as an excuse to be rude, must be engaging in 1950s attitudes towards mental illness or disability.