That's collusion and sandbagging, and I would definitely call it cheating, especially if you deliberately left spots open on your active roster. If you are a 3-10 team, eliminated from playoff contention, and facilitate the ascension of, say, a 7-6 team to the playoffs, that's totally out of the spirit of the game. It's as bad as mathematically eliminated teams making lopsided trades to shore up their friends who are still in contention (i.e. Randy Moss for Sam Hurd). That's why Yahoo! defaults allow league votes on trades.
To solve this problem, I feel like the architecture of the game should not allow you to leave an active roster spot empty if there is a player eligible to fill it--that would cut down on the use of this "strategy," since you'd have to keep IR players on your roster to ensure that you throw the game successfully; that, of course, would make your gambit evident, and likely ensure that you were not invited back next season. Which is, of course, what you deserve.