Cheating single player is only affects the player themselves. But when it comes to online, multiplayer, then cheating/codes need to be restricted. It's more from a sustaining/profitability point of view – nothing drive users away when they first log on and see everyone running around looking identical and having the best items obtained with just a few key strokes.

I guess the OP's comment of local storage was also to point out the 'flaw' since it is for a multiplayer game.