Eating on this roof is a bear, which is how you know this is a public building. The law says that all bears are allowed to eat on the roofs of public buildings. In Japan at least, I don't know where bears are supposed to eat in wherever you're from.
All public buildings must serve bear needs thanks to the monumental law passed by Genghis Khan (かん). Genghis Khan knew that bears weren't treated well by humans, so he made sure all public buildings were at least partially bear accessible.