From a game theory perspective, it's a prisoner's dilemma for the teams: in the short-run each team is better off exploiting the others by using poison pills. But in the long-run, all teams are better off in a poison pill-less world and therefore they implicitly agree (through the use of social pressure) not to use them in order to stay at the long-run optimal situation instead of exploiting short-term gains. But, as GregR pointed out, the long-run optimum for the owners is coming at the expense of the players. The players would be better off at the point where all teams used poison pills, since that would increase player mobility and drive up player values. As in any prisoner's dilemma, the only way to achieve the better outcome for the owners is through implicit collusion brought about by it being a repeated game (i.e. if the Vikings screw the Seahawks today, the Seahawks will screw the Vikings tomorrow.)
Where do you get this? The entire purpose of a poison pill is to give a new team the ability to offer a player a contract that the player's own team would normally match but for the poison pill clause. While it would improve player mobility it would drive player values down.
If I am the Vikings and I have Player X that I would like to keep, right now I can tender him at, say, a 2nd round level. That's fine, because
I know that other teams won't offer him deals at the 2nd round level because they know that I will match it. But if they use a pill, I can't match it and I will lose the player. So to keep the player, I'd have to tender him at the 1st round level to force the other team, even with the pill, to pay more than they are willing to. Thereby increasing the value of the player.
No, you don't. The tender the old team puts on a player doesn't restrict the size of contract a new team can offer the player.
That doesn't matter. In the situation where the new team likes the player more than the current team, there is no problem. The new team will offer the player more, the current team won't match, and the deal will get done. That's how players move today.But look at the circumstance where the current team likes the player more than a new team. At what level does the current team need to tender the player in order to keep him? Let's say that a new team values a player at slightly above a 2nd round tender and the current team values the player at a 1st round tender. Without collusion, if the current team tenders the player at the 2nd round level, the new team will use the poison pill to get the player at the 2nd round level, which is a good deal for them. So the current team will need to tender him at the 1st round level to keep the player.
But WITH the collusion, the new team can't use the poison pill. So to get the player, they will have to offer MORE than the 2nd round level. But they only value him slightly more than the 2nd round level, and not as high as the 1st round level, so they will not make an offer, or they will make a 2nd round offer and it will be matched.
See how the artificial protection of knowing that teams wont use the poison pill means that the current team only has to tender the player at the 2nd round level instead of the 1st round level in this scenario? That hurts the player and helps the team.