There are two basic general reasons that a person who would otherwise be willing to sell to an NPE might be interested in selling to Google instead:
1. Google will pay more, and/or
2. Google provides a quicker, easier process for evaluation and decision that means you close the deal and get actual certainty (and the money itself) more quickly and with less of your own time and energy devoted to the process.
Google's effort to established a streamlined process with a "identify your patent and price at which you are committed to sell" up front step, and a well defined set of steps with fairly short time windows leading to a final decision, attempts to address both of those. (Provide a smooth decision process on purchase at the seller's desired price.)
> In this promotion, Google's answer to that question is that at the very least, you can be sure you're not selling to a patent troll.
No, you've got that wrong: that's not the benefit to the seller. That's one of the benefits to Google (and, presuming Google doesn't use patents like a troll, potentially everyone else in the tech industry and everyone indirectly harmed by the drag patent trolls put on the technology market.)