I am not sure you understand.
I'm sure I did. You said an average player + computer would beat a poor player + computer.
There's no general principle afaict.
If the program is strong enough to easily beat both, what advantage does the average player bring? They may as well be monkeys operating the keyboard if their choice of moves isn't as good.
Of course that's not always the case, but that's where words like "average" "poor" and not specifying which computer and which program are handwaving rather than making specifics.
Computers can't play chess, they just calculate as many positions that could occur as possible. They are just very very strong tactical.
Well yeah, but before you start being pedantic you should make your mind up

Obviously computers are programmed and yes, most of the algorithms used are calculating not learning or adapative etc.
But any mention folk have made of computers playing chess is made in exactly the same way that you speak of the computer playing chess with phrases like "They are weak at strategy" or "they are very very strong tactically" - neither of which is true because they can't play chess they are just calculators

I'm sure most can take the fact that the computer is programmed for granted.
Similary, you acknowledge that computers don't always win, yet say they can "just" find the best move from a limited set by looking in a database similar to openings and endgames. If they could find "the best move" they would always win [or presumably draw against another algorithm that could pick the best move] But there is no such thing as "the best move"
Albeit you're right to note that the search space is less than go, the space is big and there is no search which will give a definitive move from which a win is inevitable from all positions irrespective of what your opponent does in response. Even if you had time to search the whole space, "best move" at that point is some weighted heuristics or similar - far from "just" searching, it's the difference between a program that beats kasparov and one that doesn't.
It might be easy to dismiss the machine as a calculator, but I think there is more complexity to what IBM did than you suggest, even if part of their success today is tin.
Actually, a lot of people think Deep Blue was helped by a human hired by IBM, and that is also what Kasparov believes.
Yes, which partly negates your argument IMO - albeit Kasparov didn't have a computer, he is neither average nor poor, so I don't see why he shouldn't have won against this human as his whinge implies that he felt stronger than the computer. [If I were the guy I'd happily take the credit]