Thanks to the posters taking the time to explain themselves on here, of course no-one (I hope!) thinks all shy people are rude! I think I can tell the difference quite easily. But it does exist, some people just don't care whether they are rude or not. And some will use shyness or, as I have heard from a few people, "not having any time for more friends" for being rude. The examples I cite upthread of people refusing to shake hands and turning away when you try to introduce yourself at a social gathering - I mean, seriously? These people presumably hold down jobs, were obviously there with families and should know how to behave in social situations. There are just some people who are unpleasant and who, as adults, have never bothered to do anything about it, no matter how badly they have been brought up. Shyness is a great excuse for this but not perhaps genuine?
I must admit I also think about a man I know when I hear shyness used as an excuse for rudeness, because in his case, it was. He could be perfectly sociable and pleasant when he wanted to (often when wanting to be seen talking to a woman in front of his family), but as soon as you ceased your purpose for him, he would drop you. As in literally refusing even to say hello the next time he saw you. He would make friends and then they would get the silent treatment and be ignored so they would give up on him. He ended up losing his job because he pissed off so many people even at work. Now, this was a man who could be perfectly charming when it suited him, and as a well educated adult from a lovely family, knew perfectly well how to behave. But he wouldn't make the effort - no-one was expecting him to be the life and soul of the party, but just to be pleasant when someone else made the effort to speak to him. I don't even think he was shy, he just couldn't be bothered with other people. I can see him ending up a lonely old man.
So what I wonder is, is how much should you make an effort, if you are shy and introverted or socially anxious, if you are able to? Because most people I would say are shy in new situations, so where do you draw the line between withdrawing and not making the effort, and trying e.g. to make small talk?
I was a very shy teenager, but when I went away from home to university and then started working, I pretty much lost my shyness. I can imagine that not working or making much effort to meet other people would contribute to shyness.