As far as i'm aware, they've signed a contract to fight for the UFC. I don't remember reading anywhere that they have to stay at the UFC till they finish. Infact I know some of the players have gone to fight in other leagues and have returned to the UFC. Frank Mir is a commentator for WEC but has signed to fight in the UFC, which is right in my opinion. The fighters sign up for a certain amount of time. Once they finished their contract, they can go sign up anywhere else if they feel the need to.
EliteXC and Strikeforce all have contracts in place I presume. Their fighters won't be automatically coming to fight in the UFC, that and because Dana won't allow it without a contract. It wise decisions.
Take Nick Diaz, he fought in the WEC before the UFC, then did a spell in the UFC and now is back at EliteXC, Strikeforce and what not. At some stage, i'm sure if he wants to go back to the UFC, he can do so but he'll have to sign a contract for that. I don't see anything wrong.
Again, how much do you think these fighters earn at EliteXC, Strikeforce, GFC etc. versus the UFC? I'd say a lot less. But that's my personal opinion.