It's interesting because some people simply don't think it's rude or personal to ask about someone's income.
I'm not saying this rule applies to the general population but in my experience, people on lower incomes and from lower income backgrounds speak more freely about income. Maybe it's because if you know you're broke, your mate is broke, and your parents are broke, what's the difference between you earning 15k, your mate earning 14k and your parents earning 17k? The differences are big enough to make a difference to personal finances, but not big enough to put you in a totally different income bracket and lifestyle.
These are the sort of salaries my working class parents have and I grew up with. So my family don't think twice about saying "congratulations on the new job, so what are they paying you then?"
Meanwhile my DH's family who are high earners are very, very private about money. They wouldn't tell their own son what they earn when they moved away and got new jobs. I have a fairly well paid job and there are a lot of high earners in my office, no-one would dream of asking their colleagues what their income is. In contrast, I've overheard retail workers in shops discussing their salary plenty of times.
Personally I was happy to talk about my salary when I first graduated, and my friend with an entry-level graduate job often tells me about her salary and her battles with her boss for a pay rise etc. But as my salary has increased over the years I now feel embarrassed to tell my friends and family what I earn because I know that they earn far less and that makes me uncomfortable for some reason as I feel that we're all equal and I don't feel like I deserve to be earning more.
So I'm tempted to think that people on lower incomes are more open about salary figures whereas higher earners prefer to keep it to themselves.
This is just my own observation though so feel free to tell me otherwise!