Crew meal
The RNR aircraft are not parked up all of the winter. They are all used on a rotational basis so that hours/maintenance are spread across the fleet. They couldn't do this is some of them were parked elsewhere as it would be prohibitively expensive to rotate them in and out of BHX.
From an airport perspective, some income from parking is better than no income; plus we can assume that RNR are a valued 'customer' and are looked after accordingly.
Regards