Don't karaoke singers actually SING? And don't they JUST sing, and not pretend to play as well?
Karaoke singers are the essence of honesty (making absolutely no effort whatsoever to hide the fact that they are merely singing over a track) by comparison to the scab who lip synchs and play-synchs, but tries to fool the audience that it IS him playing...
Karaoke is an audience participation fun thing in the far east. Only we greedy Westerners pervert it into something dishonest and dirty, in an effort to grab the Almighty Dollar.
Don't think lip synching is a bad thing..? OK, go ahead and lip synch your next show. Save your voice, turn down the arranger. Just pretend to play... You know, with luck, they MIGHT not even notice.
But stand up in the middle of the show, and to a packed house, go "OK, guys. I confess. I am merely lip synching to everything I did tonight. I didn't actually play a damn thing, and that wasn't me you heard singing or playing. Who wants their money back?"
Don't expect to make much, that night! And do it somewhere where you don't really ever want to play again