What do you know about the owner of the lock?
I ask because when I lived in Dewsbury and worked for the YOT, we had a boy with fairly MLD and Autism involved in petty crime. Alongside social services we managed to get him set up working part-time at a local garden centre, which he would cycle to, and where a lock had been left to help add routine to his day. He’d ride to work, lock his bike up with the lock, work pretty much for free, and then cycle back home. And then some prick changed the combination to his lock, and panicking he left his bike unlocked because he didn’t want to let down his work, and the bike was stolen, and he went off the rails in a fairly massive way.
Granted, this is a fairly extreme case, but your ‘joke’ is fairly hollow and meaningless. You could have put a note on it; or moved it elsewhere but left the combination unchanged. By not doing either of these things your behaviour becomes an example of dominance and controlling behaviour, which I’d consider fairly worrying if it was me…
Me? I agree with you about locks on street furniture, for the record. But I fail to see how you’ve taught anyone anything.