Nike tends to lock up the top guys. Adidas took a risk thinking they could match Nike with Hunter. (Think under armour signing young injured Steph curry) Hunter's agent negotiated smart contract 10 yrs, no cut, college, continued Tin/mom coaching.
Hasn't been a big win for Adidas, but they have to honor contract, and they still can get use from him as part of their 'stable' of sponsored athletes. Meet and greets, pr, shoe testing, motivational speaking, print ads. He's not a diva, he'll reliably handle his responsibilities.
In frame of entire Adidas running shoe budget, he's not a big cost. Win some, lose some - ya gotta stay in the game.