The Bush administration's public statements about their decision to shoot down a satellite that's already falling down anyway are so ridiculous that everyone's looking for the real motive.
There are many significant political ramifications that would happen as a result of this. The US has been berating the Chinese on their ASAT test but now demonstrate that it is okay as long as it occurs at a low enough altitude to prevent long-lasting debris and can "save lives". This is close to an implied "ok" for the US and other nations to conduct more ASAT tests, which could open another arms race. I am also certain that Russian and China would also see this as a slap in the face as they are trying to revive the Peaceful Uses of Outer Space treaty discussion and ban on space weapons. It would further negatively affect the relations between them and the US. Which could lead to increased tensions, arms buildup, etc etc etc. Nothing good for anyone outside of arms manufacturers and politicians that need a bogeyman to scare people into voting for them.The Arms Control Wonk, Jeffrey Lewis, has a great post and comments section analyzing the dangers of satellite debris creating more of a menace than the falling artificial star itself.
For myself, I think anyone looking for the real motive should be looking to the writings of the neocon think tanks that have always been the biggest proponents of anti-missile, anti-satellite and space-based weapons. They are calling upon the Bush administration and conservatives to twist and parse like a crowd of Clenises denying a new indecent liason in order to claim that "space was weaponized when the first ballistic missile was deployed, because ballistic missiles travel through space on their way to their targets." Any treaties against space-based weaponry would stop them realising Reagan's dream of Brilliant Pebbles interceptors permanently hanging over everyone's heads.
If this shot fails, it will be used as an argument for space-based interceptors instead. If it suceeds, it will be used as an argument for space-based interceptors as well.
Update The Russians aren't fooled.
Russia has accused the US of using a plan to shoot down a broken spy satellite as a cover for testing an anti-satellite weapon.
The US said last week that it would use a missile to destroy the satellite, to stop it from crash landing.
Officials say the satellite contains hazardous fuel which could kill humans.
But Russia's defence ministry said the US planned to test its "anti-missile defence system's capability to destroy other countries' satellites".
..."Speculations about the danger of the satellite hide preparations for the classical testing of an anti-satellite weapon," a statement reported by Itar-Tass news agency said.
...Such testing essentially means the creation of a new type of strategic weapons," it added.
"The decision to destroy the American satellite does not look harmless as they try to claim, especially at a time when the US has been evading negotiations on the limitation of an arms race in outer space," the statement continued.
The Russian defence ministry argued that various countries' spacecraft had crashed to Earth in the past, and many countries used toxic fuel in spacecraft, but this had never before merited such "extraordinary measures".