I do see lots of petty Hitlers who think that they are good enough to decide who is not.

You seem to be possessed of a Demonic inclination for Hyperbole of late, Richard. This is simply about appropriateness and common courtesy. Join in where appropriate (see Jim's post above) and have the courtesy to leave it to the singer to sing their song. The petty Hitlers are those who feel it needs adding to - either with their own instrumental prowess or with their dulcet vocalisings.

Don't ascribe motivation,

Forgive me (and others) for assigning motives to such behaviour but one must wonder, if only to understand what appears to be a somewhat sociopathic inclination on the part of those who, more often than not, haven't really got their act together (to say the least) or else are simply attention seeking, trying to intimidate, or even courting conflict by being plain rude. I know folk is generally a nutter-friendly zone (one of the things I truly love about it in terms of cultural / social sanctuary) but no one should have to be put in the very awkward position of having to tell such a person to shut up (often in fear of their personal wellbeing) just as no one should have to suffer such a bombastic intrusion into their art in the first place.

Nothing could be simpler.