There are no free passes like that in the US patent system as far as I'm aware. You don't have to knowingly infringe a patent to be taken to court by a patent holder and potentially held liable for patent infringement.
True, that's usually the case. However, it is still entirely up to the whim of the entity asserting a patent claim (rightly or wrongly) as to whether or not you end up with a very large legal bill. That's a huge amount of liability and personally it terrifies me.
