Of course it's always possible to turn selectors in the wrong direction even when safety devices are fitted.
However when you turn the fuel selector "OFF" after engine shutdown you are practising how to do so (as well as ensuring that it is physically possible to select "OFF"). It seems to be the practice of some operators to leave the fuel selector ON after shutdown, unless perhaps it is the last flight of the day.
I have flown with some pilots on the PA28 who were not aware of how to turn the fuel selector to the OFF position because there were not used to turning the selector to the OFF position.