Fantasy Football leagues do tons of things real NFL teams don't allow.Ask yourself, why don't NFL GMs just include as a condition of the trade that the traded players may not be used against their former team?
That'd solve the problem you've identified.
Do you think the league office would be OK with that sort of agreement?
Starting with letting regular people like us run the teams and make the rules.
My only point was it's entirely reasonable for a GM to not want to face a player he traded away and it felt like the opposite of tanking.