I would guess its because RBL have their squad for the season and didn't think they could match Us or Dortmund in the players interest.Makes you wonder why RBL weren't interested in any of them.
The release clause is there so that Red Bull clubs can buy off each other for cheap if they want, with RBL being at the top, doesn't mean they have to buy. Problem with a release clause is that other clubs can get involved and then its up to the player.

