Because SCO can go after the person who submitted the code directly. If you buy a stolen car you lose the car but are not put on trial for GTA. The person who seteals the car (or code) is the one to go after not the person (or persons) they sold it to...
Perhaps this is a good time to recall that over a decade ago, ATT sued BSD for coping their UNIX code. As the case progressed, it turned out that ATT had actually copied BSD code into UNIX. The case was settled out of court.
I do not think the present case will turn out to SCO's favor.