Why don't people seem to understand what 'Self Service' means on the locks?

[Momac]

That's for the canal system (and other canalised inland waterways) where water preservation is much more of an issue.

It's not just about water conservation.
It is quite clearly safer to leave a lock with the gates closed and the paddles down. The lock will tend to fill which is safer overnight in case someone falls in they may more easily escape.
At the town lock near me on the R.Trent the lock keepers leave a top gate slice open slightly to keep the lock full. This is specifically for the potential falling in at night of people who might have had one shandy too many.
If self operating a lock on the Trent the gates have to be shut to get the key out of the operating mechanism. Obviously not so on the Thames.