[paulgb]

Part of my assumption is that if the patent should truly not have been granted, the start-up has a decent chance of winning if they have the resources to fight it. (I do know of some counter-examples, like RIM and D2L)

The trolls don't want to enter a losing battle because of the cost (even if they're in the business of litigation, time spent suing one company is time that could be spent suing another). Also, losing would damage the validity of the patent for future cases. (In my understanding, that is. IANAL)

[nitrogen]

In my reading, trolls rarely have just one patent. They tend to buy silly, trivial patents in bulk from wherever they can. Since a single lost lawsuit could cause the troll to lose only those patents they assert in the suit, the damage to them is minimal. I think this makes trolling even more insidious -- even if you (as a startup) win, you lose.