I think Overeem ain't gone to the UFC because of this. He likes being able to fight in SF, Dream, K1 etc He can maximise his earnings.
Overeem isnt in the UFC because Dana wants ALL options on his fighters. Overeem likes to fight and makes a very good living off the K1 circuit. Why does he need to give that up because Dana says so? This is the same reason why Manhoef isnt in the UFC. SF allows them to fight in other organisations so as far as im concerned its a no brainer.