I think you're missing the point... It is bad publicity if your followers read a negative thing you say about a company. But it is good publicity if that company publicly fixes it. They essentially turn customer service into free marketing (so maybe its intentional that its the more effective than other methods of communication for receiving customer support?).

Exactly this. Sure most might go unnoticed if they ignore, but all it takes is one to go viral and really be bad. That's the kinda risk they're trying to minimize, plus yeah it looks great when you see a company "go out of their way" consistently.