The NFL cap is hard, and getting under the cap is the reason for a lot of trades in the first place - there are no exemptions to keep your own players, so for those teams, taking salary back is the last thing they want to do as it defeats the objective of the trade. Also, since there are 6 rounds in an NFL draft and a decent GM will get good players out of 3,4 or even 5, you get more trades for just picks with no player (or salary) on the other side at all.