Nokia Betalabs

After leaving the application, it does not get removed from memory, remains working and eats up the battery big times by preventing mobile data connections to drop.

This doesn't show up in the rudimental task manager; you need a better one (like jbak taskman) to see it.

I read several excuses by Noka, calling this a necessity, and other - sorry - lies.

This is NO necessity, this is a plain BUG.
So please, Nokia: Terminate the application after quitting.
It eats up memory and battery, draws mobile data for no reason.