When you have a monopoly like the NFL, the only way you are allowed to have it is to have a collective bargaining agreement with a union. In this case, the NFLPA. In that agreement, the amount of money for salaries is based on the amount of money they NFL makes. So, in this case, "basic" economics doesn't apply. Basically, the players are partners with the owners in this complex economic reality so as the money the league makes increases the amount of money the players make increases. And the reverse is also true, at least this year.
So, the amount of money NFL players make isn't based on basic economics, it's based on a collectively bargained contract that gives the players a share of profits. So, the owners don't have a right to demand more work since the players are getting more money unless they negotiate for that in the collective bargaining agreement. And, as a member of the NFLPA, Kamara is perfectly in his rights to speak out and let the NFLPA know that he doesn't like the addition of another game and if the NFL owners want another game, then he wants more money. It's all part of negotiating.