Back in the early 90s - I think it was - when email first became mainstream in business and the workplace, I quickly noticed how people behaved differently, compared with their face-to-face persona.
More recently, inhabitants of the internet have started to attempt to use emoticons in a better way, to help convey the 'spirit' of their words, not just the literal meaning. It is amazing how often words are misinterpreted in the absence of some clue to the person's intent or context, especially when different nationalities are involved.
But getting back to the OP's issue, I agree that drive-by enquirers who subsequently vanish can be annoying and this is another facet to the internet and it's new (or lack of) social etiquette. I bet the folks we're talking about, if they asked a colleague at work for a advice, would be sure to thank them, yet on the internet and dealing with strangers, they seem to lose their manners. I don't think they can all be horrid people and I suspect I might be guilty of this myself, though I can't recall. Perhaps it's just that we all tend to think of the internet as simply a resource, like a dictionary or encyclopaedia, forgetting that it's actually people we're dealing with.