@marinelife Yes, but the question is why they are considered rude.
In part, it is because Americans like to pretend that they believe work is good in itself and that they would work even if they didn’t get paid.
Sometimes, though, I think men treat money like they treat penis size: we all know that anything more than enough is just icing on the cake and that too much can actually be a curse, but that doesn’t mean there isn’t some primal “more is better” mentality screaming out from underneath. The benefit of finding out someone has less is balanced against the detriment of finding out someone has more, so a “don’t ask, don’t tell” policy is adopted. We’re happy to make suggestive displays, but not to present the unadulterated truth.
There is a danger here, however, as not talking about money can help oil the gears of corporate oppression. Why don’t employers allow their employees to discuss compensation? Because it allows them to pay people less than they are worth. If you don’t know what the fair market price is for your labor, it becomes rather difficult to demand it. The system is also self-reinforcing because it keeps people from discussing what they make in secret lest their coworkers get angry about whatever differences there may be.
Again, though, the comparison with penis size could be made. Most men feel relieved when they learn what the actual mean, median, and mode are with regard to penile statistics. Similarly, there is a lot of public information about what particular groups of people make, and that information can be useful in discovering who is and is not being paid fairly. It might be uncomfortable at first, but getting all the information out there is probably worthwhile in the long-term.