The players are paid to play football and win games. That includes the physical nature of the sport of tackling, hitting players and forcing fumbles.
But what a bounty system does is support the concept of HURTING an opposing player. That is beyond unsportsmanlike right there. Football is physical enough as it is, purporsely hurting someone is not football.
Plus you get into the whole notion of extra money to players for their play on the field. This circumvents their contract, NFL rules and the salary cap. What is to prevent an McNair from saying, "Cushing, here is your contract for $2 million per year for the next five years. But if you give Peyton a concussion or get him taken out of the game... there's an extra $2 million in cash from my pocket to yours." Yes that's grossly exhagerated but having a pool of money for players to do something circumvents the salary cap. You can't pay players anything outside of what their contract states.
What's going to be interesting is when this investigation leads to finding these cash payments being made and the players not reporting it on their tax return. Now you've got the IRS after them for tax evasion.