I suspect you know the answer.
The Seahawks invented the 12th Man concept, then found out that it was already copyrighted by Texas A&M, who has had the 12th man as a tradition since 1922.
The school allowed the Seahawks to use the moniker for a token $5000/year.
This year, the Seahawks have opted to drop the "12th Man" in favor of just using the number 12, since it is more of a symbol anyway, and no doubt to keep from offending all the "12th Women". I doubt the $5000 had anything to do with it -- they probably spend more than that on Gatorade per game.
I am an Aggie and also live in the Seattle area part-time and am both a Cowboys and Seahawks fan.