Short Answer: Yes. They can do that.

Medium Answer: There are several AddOns that have done that in the past, though from a quick skim they haven't been updated in some time (it's possible there are more up-to-date AddOns I missed). It wouldn't be hard to make a simple version that covers the general case, (i.e. pressing the keybind to un/sheathe without spamming the key).

Longer Answer: It would take a lot more effort than likely worthwhile to make it foolproof. For example if someone spams the un/sheathe key it could break. Also the game will auto-sheathe the weapon in certain instances and there's no specific "event" that occurs in-game when the weapon is un/ sheathed. One could check continuously while the game is running the status of the weapons (sheathed or unsheathed) and if it changes for any reason, but that might be a bit more intensive than it's worth.