Earlier quoted context omitted.
It's required by company policy. Otherwise the comments could be seen as astroturfing. The policy is so strict that it specifically says you need to put a disclosure in every individual tweet, a disclosure in your Twitter bio isn't sufficient. I think that part of the rule is broken frequently though.
Interesting. Is it limited to Twitter? I can see the company fearing a backlash in social medias where you're operating under your own name (and thus can be traced back to Google) but what about anonymous and semi-anonymous forums like HN?
It looks like it applies to basically any online post/comment, regardless of if it's anonymous.