In years past hats have not been turned on for private betas since "the site should focus on being born, not wearing hats". This year, we only have one beta, but it's not really a traditional beta. So we didn't really think to exclude Documentation. (Since almost all the holdouts opted in this year, the list was short: MathOverflow.) So in a sense, it's a bug and we screwed up.
On the other hand, Documentation isn't in a community-building phase, but a feature-building one. Every single person in the beta is, by the requirements of getting an invitation, part of Stack Overflow. It's a lot less likely that hats will be a distraction than it would be if there were lots of users new to the network. So I personally don't care if the feature is turned on or not.
The folks developing and testing the new Documentation feature might not want it. It's pretty much up to them to decide (or not).