the fact that they can't be proactively challenged
This is not correct. Any person can file a reexamination request and present a substantial new question of patentability with the USPTO. Here is a wiki link http://en.wikipedia.org/wiki/Reexamination. So if anyone has some publication that should invalidate a patent, by all means attack it. Also, there a declaratory judgment actions that can be used to initiate a lawsuit before the patent holder sues you. However, the patent holder needs to take some action that makes the likelihood of a future lawsuit quite high.
I'm not sure the US operates on a "loser (almost) always pays" system.
The U.S. does not operate on a loser pays system (except for court costs). Each side bears the cost of his own attorney's fees, except in the rare situation where the court awards attorney fees as part of the judgment. Also, many tort cases are filed on contingent fee structure and in those cases the attorney is paid out of the award, but technically the plaintiff is paying their own fee and merely paying after the fact with money from the award/settlement.
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