He was an UFA last year and signed with the Bills for basically the minimum for someone with his number of years in the NFL.
And releasing a player that late in the pre-season makes it much tougher for them to hook on with a new team. At best he's probably looking at being signed after game 1 so the new team can pay him game-by-game.
It's a numbers game. There are 4 true safeties (Rapp, Bishop, Forrest and Hamlin) not counting Owens, 10 CBs, 1 returner (Codrington) and 2 CB/S (Lewis and Hancock). Again if Hancock looks promising, and especially Owens...