If you sell 10 000 copies and 1 000 change hands you still have only 10 000 copies out there. This really strikes me as a complete red herring. What difference does it make if it is the original owner downloading patches or if he stops and sells the game to another and that person is downloading, same support (or a slight trivial increase).
But if you sell a million copies? Even if on average it costs $5 (number out of nowhere, obviously) to support each copy, and a million copies change hands, they now spend another $5 million on support without seeing any revenue from the 1 million new users.
True that at any one time you have the same number of copies regardless of how many times it's re-sold, but they would still potentially spend multiple times to support the same copies.
If you have an installation issue and you contact support, that time does cost them money. If you re-sell and the new user has installation problems and contact support, it costs them money again for the same thing. Keep going in the chain..
I personally doubt it's a lot of money per person. But take ~3.3 million users potentially reselling over the years, and the new ~3.3 million again reselling. Even small amounts per person add up to a lot of money when enough users are involved.
So, if you loathe the idea of essentially wasting your $40-$50 if you get tired of a game, again how do you expect a company not to loathe wasting millions when their millions of users "get tired"?
This "conflict of interest" mainly exists because we as consumers tend to be very hypocritical. We want to do exactly what we want and we hate the idea of wasting money, and yet we don't want to allow companies to have the same ideology and expect them to be OK with wasting money because, presumably, they have a ton of money to waste to begin with (even though by comparison with paying $40 for a game, most of us have a ton to waste as well).