I would expect the terms "Bitcoin" and "Silk Road" to be better correlated than "Bitcoin" and "Free Market". "Bitcoin" and "Silk Road" simply ARE closer to one another. They're both new internet things. "Free Market" is an old economic concept.
Also, let's not forget the assumption that people using the term "Free Market" on Google think and search in the same way as people using the term "Silk Road".
I find it odd that the authors remark clearly on how Bitcoin might be attractive to people that care about anonymity immediately before implicitly assuming that Google Trends accurately captures the efforts by such people to find out more. I'm sure I'm not the only one here that uses DuckDuckGo. I don't think it's unreasonable to assume that the subset of research efforts tracked by Google is representative but the point certainly deserves more attention than "hard-to-observe clientele".
Finally, what kind of conclusion is:
We find robust evidence that computer programming enthusiasts and illegal activity drive interest in Bitcoin and find limited or no support for political and investment motives.
What fraction of people that search "Free Market" are free-market enthusiasts?
What fraction of people that search "Silk Road" are criminals?