From some tests I did it looks like a few chunks around the spawn are constantly loaded.
Probably it's a legacy from the multi/single player merge. But I don't see a reason why there should be chunks loaded all the time if no player is around. If there is one please let me know.
I guess it causes the game to use a little bit more memory than necessary.
Also the entities in this area are loaded and interfer with the mobspawning. For hostile mobs that's no problem because they despawn anyways if no player is in a 128 radius. But if there are a few peaceful creatures close to the spawn the peaceful mobspawning does not work in any place in this world because of the mobcap.
I hope you understand why I think that it is a bug and just in case works as intended I would like to know why.