It's working as designed. You cannot close a Metro application. As said above it will be in suspend mode and if it's not used for a while it will release the Memory or if computer requires additional memory it will release the Memory. It's like a Android or W7P etc. So far I have noticed it works great very effective Memory management.
Unfortunately, Metro apps cannot be fully closed without using Task Manager. It works the same way as a Mac; the program is suspended but not fully closed until further use.