I wonder if they are going to block certain content or leave it up to the user to decide what is legal or not (or to ignore!)?
And how would they know what to block? They don't and they won't -- as soon as you try to police these things you assume responsibility for bad policies, since its not possible to develop ideal content filter its best not to even try.
I doubt that they will attempt to block or filter BitTorrent downloads as it would be pretty much an exercice in futility. It is impossible for a browser to make a judgement over copyright of an arbitrary file. They have never filtered website access to possibly infringing or illegal sites either.
I suspect that the wording is in relation to the recent US Supreme Court judgement in which Grokster were seen to be specifically promoting their tool as one to download infringing material. Opera (and other such software suppliers) need to emphasize the use of their software as being primarily for legally-permitted material.
From a technical perspective adding a BitTorrent client is a smart move on Opera's part: it is a natural extension to the browser which reflects what todays users want.