"Hey Bob! I found the answer to that Star Wars question we were discussing at lunch yesterday on this great site, generalsite.com/topics/starwars".

A couple days later, Bob wants to go there to check out another Star Wars question. All Bob remembers is "starwars", and fails to find the site.

Compare to "Hey Bob! I found the answer to that Star Wars question we were discussing at lunch yesterday on this great site, starwarsanswers.com".

A couple days later, Bob wants to go there to check out another Star Wars question. Bob has a good chance of remembering "starwarsanswers".

Separate domains for different topics is more friendly to users. That's probably why Google has no problem with it--as the Google guy said, its impact on users they care about.