Well, then, we have a fundamental difference of opinion. If I see someone make claims that the vast majority of homeless people are completely unhelpable because of a specific example which they have observed anecdotally, then I'll ask for empirical evidence.

Nobody is calling that person a liar. Asking someone for evidence does not necessarily mean they consider that person a liar. They may believe they are mistaken. They may be interested in understanding how they came to their conclusions. They may want to understand why someone's world view is so different to their own.

You should ask yourself why you consider asking for evidence on a discussion forum rude. It sounds like you feel it's overly confrontational. If that's the case, then you might want to consider your own confrontation (ala "If you feel so passionate about the issue, do your own research and provide your own citations against. Nothing wrong with that. But demand that the person who said something you don't like should do it for you? Rude.")