In single-player games, "cheating" is a completely subjective term that only describes doing something the player feels is unfair or makes the game too easy.
For instance, I consider level grinding in RPGs cheating, and would feel bad if I did it.
It's really just all about doing what satisfies you. If you have more fun using cheat codes, there's no reason not to. But most people want to feel a sense of achievement when playing, and that's usually lost when using cheat codes.
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