@hazelnot I don't know this for certain, but my guess is that laptops and such have a 'bypass circuit' that allows the device to run directly off mains when available, whereas in phones and such the power is always drawn from the battery which may happen to be getting charged at the same time.
If this is the case, then a laptop should work without any battery at all (which it typically will) but a phone shouldn't (which I don't know whether it's the case).