Despite that guy being an asshole (at best) - we join a lot of these websites with a pseudonym under the expectation that other posters and lurkers don't know our real identities, and more importantly our real life counterparts don't know our pseudonyms. True anonymity on the internet doesn't exist - if someone or some organisation wanted to find out who you are they can do so really easily.
However, there's still an expectation in most people's minds about the separation between forums and real life. If that wasn't the case, people would be posting their own personal details left right and centre - most don't and on many websites such as Reddit it is against the TOS and for good reason.
That means you can delve into your interest groups without fear of judgement - heck I would be very uncomfortable with anyone IRL reading my post history even though it lacks objectionable content. Especially when it goes back years, and there's stuff I may regret and find embarrassing to explain without context.
I'm not comfortable with the situation because large, private organisations doxxing posters sets a dangerous precedent - CNN definitely isn't the first to do so - but it's a very dickish thing to do. Yes, the poster's views are godawful, but doxxing isn't limited to those with godawful views, it can be used against anyone. Breitbart doxxing posters who are anti-Trump would be the nightmare 'revenge' scenario, for instance.
Unfortunately, the only reason this blew up is because it plays on the Trump narrative, and as such the right came out in force on social media. Either way, CNN handled this poorly - they shouldn't have followed this guy at all. That meme made no difference to their organisation and they've done far more damage responding rather than shrugging it off.