If it was up to me, entering the Konami code would unlock everything in every single player game. Everyone has the right to access content they've purchased in the way and order they see fit.
But when you're playing with other people there's really no excuse.
Consider games like WoW. Ask anyone what the game is like, and you'll find that the game "doesn't really begin" until you are already max level. That is, the "real" content of the game is all of the Guild-related activities. Everything up to that point is more or less an extended tutorial that lasts for dozens of hours.
Cheating because you want to skip the "tutorial" and quickly get to the point where you can play at the same level as your friends seems like a perfectly reasonable goal. In fact, now a days I believe WoW provides a free (high) level character for all new players, but for a long time, you needed to run through that grind yourself, or pay a microtransaction to skip up.
What I do in fact fundamentally not understand is why games are designed this way†. Good on WoW for apparently letting players skip it nowadays. I admit, I'd have trouble faulting someone who wanted to cheat their way through.
† Outside of games that explicitly want to sell micro-transactions. I understand that all to well.