In Spain, release clauses are activated by the players only. Since the players don't have millions of pounds lying around, it's usually the clubs who pay for them by lending them the money. When foreign clubs are involved, the money they lend to the players is taxed because it's from outside the country. I believe the usual practice is that the two clubs just settle (by not activating the clause but still agreeing to a figure close to the clause amount), but Atletico don't seem so keen. They are probably hoping to get a few more millions out of us by agreeing to not use the release clause (thus, saving us the tax money)