Dear Moley, remember our immortal Winston: "But in the morning, I will be sober and you will still be ugly."
(That Moley should choose to have a go on a thread about musician rudeness - the irony! :-)
One thing that hasn't really been touched on is the question of why the non-players are there in the first place. If it's a semi-formal singaround or session, or a formal folk club performance, then anyone there should know the score - namely STFU or at least keep the noise down if you do need to talk. And amplification here makes sure that people at the back can hear as clearly as people in the front row.
But if you're in a busy pub and other people in the pub have come there to have a chat and a pint with their mates, it's a bit of a different situation. The funny thing is that most people *are* still listening, so if you start up something unusual then you might suddenly find the room goes quiet as everyone stops to listen. The mistake then is to think you've got them and they're all yours - actually the noise level is likely to increase afterwards, as everyone starts talking at once! :-/
The big mistake of course is pushing your voice to try and respond. If people are listening, they're listening. If they're not particularly interested, then "SHE! LET! GO! MY! HAND! AND! SHE! MOVED! THROUGH! THE! FAIR!" isn't exactly going to change their mind. Even so though, learning that there's a time and place for delicate finger-picked arrangements and there's a time for balls-out strumming is a good lesson. Amplification in this kind of setting just lets the former be audible - it doesn't mean it's going to increase the number of people interested in what you're doing.
You can lead a horse to water but you can't make it sing (or listen to) folk music...