Well most of the time when a band signs a contract with a label it agrees to give the rights to the songs they record for that label to the label, or at least that's the case in the metal world. So they never lost the rights to the songs, they just never had them themselves, the label that they were on at the moment of the releases did and does. This is one of the reasons why Jon Schaffer doesn't do a DVD yet, Century Media still has the rights to their songs I think. Dunno if he's ever going to get them. Would take some serious cash.
But I think that it wouldn't have killed Opeth/Music For Nations to pay this and that percent to Peaceville and Candlelight. It's just that Opeth couldn't be bothered to fight a bit for getting the old songs (Mike has said quite a lot of times that it was easier to just play the new stuff) and Music For Nations of course only wants the stuff to be played that Opeth has recorded for them, and therefore doesn't willingly pay the old labels. Music business sucks.
Goooooodnight.