Often the the shell companies don't actually own the patents, they just license them from the parent company. This way when the shell company loses and goes bankrupt the licenses is automatically revoked and there is nothing left to claim.
Patent trolls might be evil, but unfortunately most of them aren't stupid.
The parent company simply writes that into the license. Companies like Intellectual Ventures set up a shell company and give that company a time limited exclusive right to some patents, including the right to sue other companies for infringement, during that time period. In exchange for this exclusive license the shell company agrees to pay, let's say, 95% of any revenue generated by sub-licensing or court settlements to the parent company.
Incredible that such a transparent strategy would not be punctured in court. Just requiring that the principal holder of a patent is the only one with standing to sue for infringement would get rid of this particular trick immediately.
Yeah, that's why one of the major target reforms is being able to track the ownership chain up to the actual patent owner. This would allow courts to reward damages directly from the actual perpetrator to the victims of patent trolling, rather than this shell game nonsense.
Presumably what would happen in that case is that the parent company would sell the patent to the shell company, who would then just sell it back as soon as there was any threat that a victim was going to fight it in court.
It's still an improvement, but I'm sure that there are plenty of even cleverer ways to get around this than my off-the-cuff guess.
If the shell company sues ten companies over a patent, and one of them threatens to sue, and they sell the patent back to the parent company, wouldn't that mean they no longer have standing to sue the other nine either, and they'd have to file a motion to dismiss all the rest?
Could the company being sued start playing the same game? As soon one is sued by a troll, reconfigure the 'offending' bit into an asset-less shell company that you purchase/license/etc. things from.
Patent trolls might be evil, but unfortunately most of them aren't stupid.