Washington could not trade him until he signs the franchise tag, and if Cousins wanted, he could delay signing it for weeks or even months, with the Redskins having to count his approximate $34.5 million against their salary cap. (HINDERING THEIR OWN ABILITY TO SIGN FREE AGENTS)
Cousins also could tell any of Washington's potential trade partners that he is unwilling to sign a long-term deal there, dampening that team's enthusiasm for surrendering a top draft pick for his services and holding up any potential trade.
If Washington decided to pull the franchise tag to get Cousins' contract off its salary cap, it would lose the right to recoup a 2019 compensatory draft pick that it would get if Cousins were able to leave right away as a free agent. So tagging Cousins would undoubtedly carry risk.
http://www.espn.com/nfl/story/_/id/22315705/washington-redskins-consider-placing-franchise-tag-kirk-cousins