Anyone who expects silence from an audience that's been drinking needs to be in a different line of work, or a different venue.

Singing along uninvited during a concert is always rude, chorus or no. Do you really think everyone came there to hear you? If you don't want the musicians to make that decision, keep to sing-a-rounds and sessions where the musicians haven't gone to some trouble to prepare a song for performance. At least make sure you know the acceptable local behavior. Assuming that a folk song SHOULD be sung along with because it's a folk song is failing to recognize that folk songs often get played these days in places that are not community singing opportunities.