I could site many reasons why it needs to close down, but I it would be infringing on our intellectual property.
You asked the question, I gave you the best answer possible. In the light of unexpected behavior it is best to close down the application. The game is not responsible for the Internet connection and never will be. It is a game, not an operating system.
Now, if you do not like the answer, that is fine. But that is the way it works and it will not change for the foreseeable future.
EDIT: You cannot stay logged in, if you do not have an Internet connection or the connection to the server is down.