But iOS isn't running on a real computer that has a either a large battery or a mains power source, it's running on a small device with a battery that is expect

But iOS isn't running on a real computer that has a either a large battery or a mains power source, it's running on a small device with a battery that is expected by users to last at least a day. iOS' current backgrounding APIs strike a fairly acceptable balance between functional background tasks and battery life. Let's say if Apple took the reigns off background in iOS, we'd see some app developers ravage the battery and see users having terrible device experience.