I have seen many cases of audience participation over the years, with many excuses given, although none stand up to a thorough medical examination...(bend over, cough)
For artists, it's either laziness, "you've paid over the odds to see me, now you can sing the fucking songs as well" or an inability to hit the high notes/remember the lyrics, "I was double-tracked and auto-tuned on the album, so I clearly need help".
For some of the audience, it's a chance to show the poor, unsuspecting fools standing nearby that you're a real fan,'cos you know all of the words to all of the songs and that's why everyone has bought a ticket - to listen to you, you twat.
"We're recording tonight's gig" - a phrase that strikes fear into the heart of many a concert-goer; the band will be playing it safe - "can't risk the new songs or any with time-signature changes, we're not that good" - but far worse is the reaction of part of the audience, who are now going to whoop, holler and other non-English words that describe the embarrassing noises they will make before, after and often during every bloody song - presumably so that a year later they can play the album to their friends (what friends?) and point out their contributions.
Fortunately, this habit is starting to die out, for two main reasons:
i. less artists are releasing live albums - why bother with the expense and effirt involved when a reissue/best of/deluxe box-set will suffice
ii. the prats who would have previously screamed out inanities during the set are now too busy filming the whole thing on their phones, so they can ...er...I'm not sure what - will they ever watch the video of the show they could have experienced live if they hadn't been too busy holding the camera high in the air? I suppose they can show the film to the same friends who earlier enjoyed their contributions to live albums.
NB John Cage's "4'33'' Live" has never made it past the recording stage, despite countless attempts
As a footnote, rock fans aren't entirely to blame - their parents used to clap politely at the end of every jazz solo played at Ronnie Scott's and other such establishments - however brief, bland or overblown they might have been - and they still bloody do.
editor's note: some or all of this might not be true