Wikipedia indicates Jonas Records may be defunct, it certainly wasn't ever Warner Brothers. SNSD is with Interscope, 2NE1 is signed with Will I Am's label.
If you have to sign with a 2nd tier label because you are basically an indentured servant to the Korean label that's been training you for 6 years your odds of being a huge breakthrough success in the US are going to plummet.
My point is that there isn't some kind of contractual obligation that prevents K-pop artists from signing with foreign labels for activities in those countries. If you look at Japan, you can see dozens of K-pop artists who've signed with local labels who have no connections with the Korean labels that those artists are signed with.
If K-pop artists have failed to sign with any major US labels, that's because those labels analyzed the artists and determined that they would not be successful enough to warrant signing them. Interscope is a major label, is it not? So why hasn't SNSD succeeded? See these posts for some clues:
No connections? Virtually all activities everywhere are carefully planned by the Korean labels. These groups can't even crap unless they have their handlers' permission.
What actually happens on the ground is besides the point. Once again, I'm referring to contractual obligations. There is nothing stopping a successful K-pop group from signing with Interscope, as you yourself pointed out. The issue is that American labels are not interested in them, because there's little chance of them succeeding.
And why would JYP Entertainment or SM Entertainment refuse to allow a successful group like SNSD to sign with an American label, if that's what it takes to achieve success? Like you said yourself, success in America isn't possible unless you sign with a major label. So it seems like that's exactly what they'd do (and exactly what happened with SNSD). Yet SNSD hasn't succeeded in America. Care to explain that?
If you have to sign with a 2nd tier label because you are basically an indentured servant to the Korean label that's been training you for 6 years your odds of being a huge breakthrough success in the US are going to plummet.