Things like this are actually done for very valid reasons, but these easter eggs are a really neat way to do it.

The reason? making an accurate map from a territory is (or used to be) difficult and takes time. Introducing fictional stuff in a map is a way to:

- figure out which of your cartographer competitors are copying you

- bring the case to court (factual data isn't protected by copyright, fictional data is).

Even Google Maps add a few fictional elements, but they're much more boring, like adding ghost streets in rural areas.