I think being rude is often conflated with being truthful because both are generally socially unacceptable, and thus, they tend to run together.
If there's one thing I've learned while growing up as an introvert, it's that social life, and by extension, human life, is full of lies and posturing. Interacting with people successfully requires a baseline of acting skills and energy that I don't have (and don't care to acquire). Oftentimes I'm seen as rude or cold or unenthusiastic, simply because I'm not 'putting on the show' like everyone else is. Humans generally don't want truth, not even the truth about you. They want what they expect, and part of becoming a mature adult is learning not to get upset when you don't receive it. ;]
And I shouldn't have to explain why rudeness is socially unacceptable. So, it follows that there will be plenty of instances where people get the unfortunate truth and misconstrue it as a 'rude' attack, or assume that because people are offended by something that they just 'can't handle it', and not that it shouldn't have been said. People think things like that all the time, so it's not surprising that a certain segment of the population believes that the truth should be rude, and that if something is mean, it's probably true.