The main reason I say that is because calling it "cheating" is a bit of a loaded term. To me, cheating would be breaking the rules or fudging the dice constantly behind the screen to get the results you want.
Does that mean that a single instance of, say, a player using a loaded die to guarantee a 20 is not cheating, because a single instance cannot be "constantly"?
I find this requirement that it be a long, sustained, pervasive pattern unwarranted. Even a single instance of deception or misrepresentation can qualify as "cheating." And yes, it
is a loaded term. That's kind of the point. It's highlighting how there is a shell game being played here, that things are being presented
as though they were something that they aren't.
Consider this: if adding extra enemies to an adventure is cheating, then every time a DM has rolled on the wandering monster table, they've been cheating! These monsters are presented as being in addition to the other monsters already present, and very rarely does an adventure (or monster manual, as they've had terrain-based wandering monster tables in the past) say "oh by the way, each of these results may only be encountered once").
But that's not what the random monster rolls do. The random monster rolls
replace encounters that had to be built by hand. It isn't the GM
choosing to insert monsters that weren't there; it is them correctly using the encounter design and creation tools. That's like saying that rolling damage is cheating; it
cannot be, that is how non-fixed-damage things work.
Inserting extra monsters that weren't there or rewriting the stats of monsters that are there, simply because you
feel like they should be, not because you're using the rules, however, would not be using the rules as designed. Like...pretty much literally not that.
It's sometimes written right into the adventure that infinite monsters exist, such as Undermountain, where in addition to the many gates that things could blunder into, Halaster has been known to kidnap monsters for his dungeon, and his apprentices have even created monsters that wander the dungeon!
Then they have, as I said, done the work to explain that this is possible. Have they also done the work to explain why these allegedly infinite monsters don't just destroy the players instantaneously? Because that's kind of the double-edged sword here. How can there ever be safety enough for even fifteen minutes' rest, let alone eight hours?
Because the DM creates the world, they could easily write the ability to add monsters out of nowhere in it's lore (hey it only takes one Fiend on the Material Plane 24 hours to start summoning more of it's kind, you know!), and this is true for canon worlds as well.
But do you not see exactly why this
isn't done? Beyond the above problem of "okay so...how is it that the opposition doesn't just
win everything forever?" you get the problem of negated stakes. An enemy with infinite, always-accessible, always-refillable reserves cannot
lose anything. Infinity minus one is
still infinity. No amount of (finite) subtraction or division can turn the concept of infinity into any finite number, let alone zero. In using such a nuclear flyswatter, you have created a dramatically worse problem in its place.
To me it feels like cheating, but I'm not going to outright say it is, because from another point of view, it's simply a tool in the DM's toolbox. And a tool isn't bad or good innately- it comes down to how it's employed.
So a player secretly bringing loaded dice is not bad or good, it comes down to how those loaded dice are employed?