A patentable invention must be novel, not obvious, and reduced to practice.
A "patent-troll" is a pseudo-inventor who obtains a patent on an obvious (to practitioners, if not to patent examiners or to judges) application of a known concept in a new context, and then just waits for engineers, programmers or business managers to do... the obvious: apply the same known concept in the "patented" context. At this point the patent troll will sue for infringement of the "patent."
Back when "non-obvious" first became a criterion for patentability, its meaning was objective and clear. "Obvious" means "not requiring the induction of a new concept." "Non-obvious" means "requiring the induction of a new concept." The applicability of an existing concept to new contexts is part of the concept of "concept," going back to Aristotle or before. A patentable, non-obvious invention was one that required the induction of a new concept.
When Pragmatism took over American law, this changed. The existential import of Pragmatism is the denial of the applicability of concepts to existence. The objective definition of obviousness was replaced by a vague sort of "I know it when I see it" ("it doesn't seem obvious to me") non-objective intuition on the part of patent examiners and judges. Hence "patent trolls."
Why is this important? Because the existence of patent-trolls is being used as an argument against intellectual property as property. And it is NOT a valid argument against intellectual property. It is, however, a valid argument against Pragmatism, and especially against Pragmatism as a "Philosophy of Law."