It's my understanding, and the GP who read the whole thing states, that the plaintiff pays the defendant's fees only if the sued IN BAD FAITH.
Close, but not quite. "Bad faith" has a specific meaning in law. It requires malicious intent. The burden of proof is on the party claiming bad faith, and it's hard to prove intent.
That language was fought over and amended. Originally, it was loser pays almost all the time. Here's what passed the House:
(a) Award- The court shall award, to a prevailing party, reasonable fees and other expenses incurred by that party in connection with a civil action in which any party asserts a claim for relief arising under any Act of Congress relating to patents, unless the court finds that the position and conduct of the nonprevailing party or parties were reasonably justified in law and fact or that special circumstances (such as severe economic hardship to a named inventor) make an award unjust.
That's a presumption that the loser pays, but the loser can try to convince the judge that their position was "reasonably justified in law and fact". The burden of proof is on the loser. and there's no requirement of bad intent. It's quite possible to start an infringement case and find out during litigation that the position taken was not "reasonably justified in fact".
The effect is to place inventors at risk of losing several milion dollars should they try to assert patent rights against a big company. It's all about making inventors afraid.