Quoting NEWS.COM: They are shooting it down because:
President Bush ordered the action to prevent any possible contamination from that hazardous rocket fuel on board, and not out of any concern that parts of the spacecraft might survive and its secrets be revealed, officials said.
The fuel tank is believed sturdy enough to survive re-entry, based on studies of the fuel tank that fell to earth after the shuttle Columbia disaster in 2003. Officials said that the slushy frozen fuel would have then been released wherever it came down.
“This is all about trying to reduce the danger to human beings,” said James Jeffrey, the deputy national security adviser.
Of course, we believe every word of it!