Lip-syncing is also a solid form of performance and can actually entertain more than you think (rupaul's drag race, comedy central's lip sync battles) which pretty much fufills the artist's job - to entertain - so why not!
I don't think those who pay to see performances and concerts where lip-syncing is involved are ripped off since what even is 'real' anyway - ie accusations that vocals on Britney Spears' records aren't even hers, auto-tuning on records, the entire process of music producing and editing.