If you think he was arrested for posting a question about Tor to Stack Overflow, then you really have a lot of catching up to do on this story.
And making people feel that simply asking questions about Tor will put them in the same category as hardcore criminals is just wrong. We're not all heartless criminals. In fact, most just want a bit of privacy.
1) They were able to find code identical to the SO question within the SilkRoad source code
2) They already had circumstantial evidence linking him to the site, this just strengthened it
3) "frosty" is the user/computer name used to generate the private ssh key on SilkRoad's server, further linking him to the site