The main reason is because the other labels would still want to sue.
Now why can't they all just buy grooveshark together? Because we have anti-trust laws that prevent them from working together like that.
So why don't they just work with grooveshark to get the licensing deals they want and capitalize on the huge userbase? Because the labels see it in their best interest to keep this grey area dangerous to enter. If they struck up a deal with grooveshark more people might get into the game seeking to grab the music industry by the balls and that's not what the RIAA wants. They want a firm grasp on everyone elses.