One line you could draw is whether there's a prior relationship between the patent holder and the defendant (or the successors to those parties). BUT FOR Lonnie

One line you could draw is whether there's a prior relationship between the patent holder and the defendant (or the successors to those parties). BUT FOR Lonnie Johnson's work, Larami (and by extension, Hasbro) could not have made Nerf guns or Super Soakers (or at the very least, it would have taken some longer to come to market). In contrast, you can't make any similar statement about what IV, Lodsys, or other trolls do. For the most part, business don't find out about those patents until after a lawsuit has been filed. There's no benefit derived from what IV or Lodsys "invented" and that's why it seems unfair. This is basically the idea behind an "independent invention" defense -- that is, if I can prove that I independently invented something without assistance from the whoever previously patented the invention at issue, I should be able to get that patent suit tossed out. Unfortunately, that's not what the law currently says. Moreover, thee's a fair amount of legitimate pushback against the idea. It's hard (but not impossible) to prove someone knew about a patent before they made something. And the question of whether something was independently invented usually isn't a binary one. If the patent holder tells his friend who tells her friend who mentions something in passing to me which inspires my invention, did I invent my product independently of the patent holder? Hard to draw a bright line there (but not impossible).