Its a very small chance of a catastrophic fire - versus the far more likely health problems to be caused by the stress and fear associated with putting people in temporary accomodation, possibly with repeated moves, that is unlikely to be particularily comfortable.
Its a similar situation to the FUkushima evacuation - where it now appears the populace suffered more health problems from the stress of the evacuation than would have been suffered from radiation had they simply remained in place.
EDIT:
A fire on the top floor is not that bad in terms of safety of life.... after all it would not trap anyone and fires advance downwards considerably more slowly than they advance upwards.
THe worst case is a fire on a floor just out of reach of the fire brigade's primary hoses.
There may only be a very small chance of a catastrophic fire but it's a chance that is just not worth taking.
If there were a fire on the top floor the melting cladding would ignite the panels below.