Not sure how to change it, but I think I know why.

When you lock, the time until the screen turns off goes right down to a couple of seconds - you can stop this by wiggling you mouse or pressing a button like normal. This is different to sleep where it uses very little power and the state is moved to memory - background process will still be running when the screen is off.

You may be able to adjust this in the GUI by changing when the screen goes off in settings, but I have not tested.

If you wanted to do this and make it default, I believe you should be able to do so using userChrome.css. You won’t be able to change the text, but you can remove the old menu item.