[N3WBI3]

But IBM is the one who copied the code (supposedly) into the kernel, Linus and the community as a whole would be happy to remove it and that would slove the problem.

What this article does not mention is that much of the copied code is being challenged to be covered by IBM Patents..

[Norman Conquest]

The most likely explanation: Code that IBM created and returned to the Unix community as a contribution was ALSO contributed to the Linux community. SCO objects, arguing that IBM's Unix license prohibits them from doing this. That's why they've sued on breach of contract, not on copyright infringement.