Well, looking through the article it seems as if the government wants Reddit to cough up the identities of these 5 people because they've spoken about or otherwise implied knowledge of who the administrators were on a drug trading operation. Now of course there might be prosecutors intending to charge them as accessories, but it might be as simple as wanting to compel testimony about the admins.
For example, Suppose you sell servers, and someone calls you up one day wanting 30 of them. You hit it off with this client, a friendship develops, and 6 months later that person, in an unguarded moment, says 'heh, I bet you never thought those 30 servers would end up as part of the 'Drugz, maaan' empire.' Now, unless you're a mandated reporter nothing criminal has occurred so far - you gained knowledge of criminal activity, but your friend hasn't ordered any more servers from you and so you're not taking part in any conspiracy, plus you were paid long before you learned this information (let's assume you can prove all this). However, if you bragged to someone else that you knew who the boss of 'Drugz, maan' is, then prosecutors would be able to subpoena your testimony whether you wanted to give it or not.
This is just a made-up example; I just want to point out that it's not necessarily a question of criminality or free speech. The government (ie the US government, following common law principles)has always been able to compel testimony except from a defendant or a small class of other people who have an intimate relationship with a defendant (doctor/attorney/clergy person). Even though a witness may object to being called and not want to help the prosecution at all (ie a hostile witness), there's still an obligation to testify truthfully. So, free-speech advocates, just because you're allowed to say something and you may not be putting your own freedom at risk, be aware that the subject of your speech could be impacted by your statements.