I've wondered the same thing before. We had a tornado rip through our town last Feb, and the American Red cross and Salvation Army were here for weeks to keep food and clothing available for those who had lost everything. I saw first hand how much help they provide. And now I donate money when I can because I realise how helpful their services can be in a disaster and they have to have donations to keep the work going.
I also feel that if any Witnesses had been victims of the tornado they would not have turned down help from these charitable organizations.