They'd still need somewhere to stay unless they are setting up their house in one day and all utilities are turned on as soon as they move in.
In all fairness, they cancelled a month before arriving allowing you a whole month to rebook. Yes, it sucks, but if AirBnB sided with them already, what honestly can you do? Its not as if they found a cheaper place.
Either way according to the strict policy they should be refunded 50% of their booking and any cleaning fees. Are you stating they received a 100% refund?