I think the idea of a high rise fire fighting helo is not so much to put the fire out but to contain the fire enough to allow the occupants to make it to a safer place, if that is a roof top for helo evacuation so be it.
You also need the building to have flat roofs or helo pads, people could of come off the world trade centres if they had pads on top.
In Australia many of the highrise on the Gold Coast were originaly built with helo pads on top but as CASA and others refused to allow there use they are now covered in antennas and satellite dishes.