When you're more encouraged than discouraged to play by the rules is when things work properly. I have never been cheated by my lightning partners, have you?
You don't have do it on purpose. Accidents happen. Someone could run an old backup and close the channel using their (outdated) commitment transaction.
oh and watchtowers can only intercept and change ownership if you trust them to have the keys to do these interceptions on your behalf..
Watchtower do not hold any keys. They only store penalty transactions in an encrypted format. See
LND's altruist watchtowers.