O, i agree, this is legal. or it should be.
The only thing I would have a problem with is if someone picked up and then dropped a bunch of players so that they wouldnt be available to be picked up till after sunday. If you are going to pick someone up to block someone else from getting them, you should have to keep them on your roster till sundays games are over.
There are many ways to improve your standing in FF.
Last year, in week 13, i purposely sat a few starters so that I would lose. I had already clinched the playoffs, and the guy I lost to was fighting for a playoff spot with one other team. I felt that due to schedules, I would rather face team A than team B in the playoffs, so i let team A beat me so that he got into the playoffs and team B didnt.
Is this questionable? probably. Thats to bad though, its all about winning. I have also on many occasions thrown games towards the end of the season to change my playoff seed, so that I could essentially choose my first weeks matchup.