The problem is that given the current state of the patent system we have no particular reason to think the patent is valid. Back before I got there a student at my dorm in college decided to wire up our dorm's laundry machines to a webserver, and we appeared on Slashdot for that. Shortly after a company applied for a patent on the idea of doing that, they sent a cease and desist letter to the dorm, because some lawyer at the company didn't realize where their product came from.
Thats just an illustrative case, but getting a patent that covers already existing products is far from impossible for a big company with lots of resources to throw at the problem. This means that, in practice, big companies all have stocks of patents that they can use to possibly shut down their competitors businesses depending on what the juries decide. But because their competitors are in the same position Mutually Assured Destruction holds and nobody actually wants to sue - especially since there's a chance that any patent might be invalidated if it came to a lawsuit and then you wouldn't have them for protection any more.
So here we have a system that isn't optimal, but its workable and stable. But then patent trolls get into the mix and things get more complicated. Since the patent troll doesn't have products of their own there isn't any threat of MAD, so they're free to extort money from companies. Now, if they actually ever had to bring a lawsuit to conclusion there is always the chance that the patent might be found invalid thus, preventing the troll from using it to sue more companies later. But if they can keep settling for just a bit less money than would cause the victim to fight the suit you can keep extorting money for a long time. But if the potential victims realize what's happening and legally target the patent - which is sort of like whats happening now.
EDIT: Sometimes, often actually, patents are actually for substantial origonal inventions where the patent aplicant actually _was_ the first to invent something. Those tend not to show up in this sort of legal action, however, and sure enough Google and Microsoft were able to find plenty of examples of prior art.
Thats just an illustrative case, but getting a patent that covers already existing products is far from impossible for a big company with lots of resources to throw at the problem. This means that, in practice, big companies all have stocks of patents that they can use to possibly shut down their competitors businesses depending on what the juries decide. But because their competitors are in the same position Mutually Assured Destruction holds and nobody actually wants to sue - especially since there's a chance that any patent might be invalidated if it came to a lawsuit and then you wouldn't have them for protection any more.
So here we have a system that isn't optimal, but its workable and stable. But then patent trolls get into the mix and things get more complicated. Since the patent troll doesn't have products of their own there isn't any threat of MAD, so they're free to extort money from companies. Now, if they actually ever had to bring a lawsuit to conclusion there is always the chance that the patent might be found invalid thus, preventing the troll from using it to sue more companies later. But if they can keep settling for just a bit less money than would cause the victim to fight the suit you can keep extorting money for a long time. But if the potential victims realize what's happening and legally target the patent - which is sort of like whats happening now.
EDIT: Sometimes, often actually, patents are actually for substantial origonal inventions where the patent aplicant actually _was_ the first to invent something. Those tend not to show up in this sort of legal action, however, and sure enough Google and Microsoft were able to find plenty of examples of prior art.