Piracy (and Anna's Archive)'s mission is to share information, especially culture, with everyone, regardless of their ability to pay for it and regardless of the geoblocks. Keeping the service hidden may benefit you and the few people that know about it, but it isn't the purpose of these sites. They felt they were protected enough, and they decided to take another step towards their objective, that's it.

In practice, nothing's gonna happen. They already have 4 different domains. Even if they managed to seize the servers and cancel every domain, all of Anna's Archive data is out there on public torrents, and their software is also FOSS. Anyone can make a mirror.