They lost Bennett to the Bucs in the process.
The Bucs claimed Bennett off waivers, which gave Bennett no say in the matter. A week earlier, the Seahawks made room for Williams on their roster by releasing fullback Justin Griffith. Griffith's status as a veteran protected him from waivers, allowing Griffith to re-sign with Seattle after the game.
I haven't asked the Seahawks whether they were intent upon re-signing Bennett. Their willingness to expose him to waivers means they were prepared to lose him, however. Bennett made a positive impression during the exhibition season. Seattle has good depth on the defensive line.
Update: The 49ers also claimed Bennett, but the Bucs had priority.