Patent infringement isn't restricted to manufacture: "Except as otherwise provided in this title, whoever without authority makes, uses, offers to sell, or sells any patented invention, within the United States or imports into the United States any patented invention during the term of the patent therefor, infringes the patent."
On the upside, you can scarcely breath without infringing a patent these days. I doubt a patent troll can operate in a non-infringing manner should a sufficiently large portion of the patent-holding community decide to get pissed off at it.
Problem is that the trolls also have few or no assets, other than their portfolios, that is.
17 USC 271(a)
http://www.law.cornell.edu/uscode/text/35/271