I can see most of the “Did they let you out of the nursing home, Grandpa?” and “Why the fuck do you care if someone cheats in/mods a purely single-player game?” comments have already been made, so I’ll move past that and mention that computer games have evolved considerably in the last few years to the point (especially post- Half-Life) where they’re really interactive movies and as such are very much now (IMHO) more about the journey than the destination.
As for Mods: Most of the mods I’ve used have been to fix gameplay imbalances or to restore features from previous games in the series removed in sequels. For example, in the 1994 game Colonisation, you could “abandon” a colony which turned out to be indefensible or in a really crappy location, provided it didn’t have a stockade. This “feature” was removed from the re-make Civilisation IV: Colonisation, but has been restored by Mods. Actually, there’s a comprehensive, balanced Mod for C4C which makes it an incredible game and fixes pretty much every criticism anyone has ever had with it, but that’s by the by. 
In Most RPGs, your character will find a lot of Stuff lying about the game world. In Ye Olde Days, most of the Stuff your character found would invariably turn out to be useful (or even vital) much, much later in the game- to the point where, in early RPGs, it was entirely possible to find yourself at an impasse because (for example) you didn’t collect the pocketwatch from the jeweller’s back in the Town Square section several play-hours previously. So, rather than restore from a previous save and have to re-play half the game, you could just open up an Item Editor programme and magic the Pocketwatch into your character’s inventory, removing the “roadblock” and allowing the game to continue.
Contrary to what the OP seems to think, most people using cheats in single-player games are not looking for an “I Win” button. So, in the above scenario, most players would not simply open the Item Editor and magic themselves every. single. item in the game. It’s there as a form of “Lifeline” to help the player progress through the story.
In modern RPGs, however, the “forgetting to collect a vital quest component at the start of the game” trope is (more or less) redundant and instead game worlds are now full of Stuff that is completely and utterly useless, but looks useful- and therefore your character might very well find themselves lugging about Oddly Shaped Rocks and Steering Wheels From a '67 Chrysalis Highwayman because they look important- but they aren’t.
So, there are mods in place to allow your character to carry more- or, even better, actually use all this Stuff to make useful things. It’s about enhancing the in-game experience, not detracting from it.
The game designers want you to enjoy the game. And in something like Fallout 3 or Oblivion, it’s not enjoyable to be getting your ass handed to you at Level 3 when you’re constantly getting attacked by Raiders with SMGs or Whirling Banshees With Fire-Magic and the best you’ve managed to get is a 10mm pistol with one spare clip of ammo, or a Rusty Dagger Of Not Doing Very Much Damage.
My policy is to play the game “properly” through the first time, with reference to a Walkthrough in areas where I’m really, really stuck (say, have spent 15 minutes of Real Time wandering around and around in circles looking for the Cavern Entrance, when it turns out that there’s a switch concealed in the rockpile next to to the dead badger that needs to be activated to reveal it), or perhaps a quick check of a Components List to see if that Fusion Battery I’ve picked up (having not seen many) is actually used for anything or if it’s just in-game “colour”.
Having completed the game properly, on re-plays I’m going to explore and want to sightsee (and do quests I missed the first time around), and to that end I have no problem with anyone wanting to give themselves better starting equipment or more ammo or anything like that.
I view meta-gaming (walkthroughs, save/reload etc) as being very similar to watching a movie, going “Hmm, I don’t think I completely understood what the Director was getting at there” and going onto IMDB or Wikipedia and discovering that the scene with the Aardvark and the Millkman represented the Milkman’s fear of change and desire to be strong about… whatever it was that was happening.
It’s not “cheating”, it’s adding to the experience and enhancing my enjoyment of the subject. As long as you’ve made an attempt to do it “properly” first time through, I see no problem at all with mods, cheats, and meta-gaming in single-player games.
Multiplayer is an entirely different kettle of fish, of course, and rightly so.
Incidentally, playing a game on “Easy” is not cheating, and anyone who says otherwise, IMHO, is grabbing at straws in a vain attempt to make themselves feel vaguely superior to everyone else.
