The way I understand it these donations go directly into trusts/foundations, which have a certain charitable aim, but these trusts are still controlled by the families of the donor, even generations later. The proceeds of the donation are then used for the charity, but normally the capital stock is not touched, so only dividends and interest are spent. The family of the donor still have voting rights of the underlying stock/ companies, so they have control but no proceeds, although in general they can also be employed by the foundation and be given a salary (within reason though).
So while these billionaires give away half their fortune, they still have complete control over it. Mind you they could also use a trust/foundation construction to circumvent inheritance taxes (surely the democrats will put them back into place in the future?) and give next to nothing to charity, while still insuring that their fortune is there for their family. In the end its not completely altruistic, but I think it is a good thing.
So while these billionaires give away half their fortune, they still have complete control over it. Mind you they could also use a trust/foundation construction to circumvent inheritance taxes (surely the democrats will put them back into place in the future?) and give next to nothing to charity, while still insuring that their fortune is there for their family. In the end its not completely altruistic, but I think it is a good thing.