http://en.wikipedia.org/wiki/District_of_Columbia_v._Heller
I feel it’s worth linking to this excellent Wikipedia article. I’m sure that the Obama Administration wants to overturn this decision because it’s an anathema to their agenda of limiting or eliminating privately held firearms. To them only the Government controlled military, the police and their private bodyguards should possess firearms and ammo.
This is a major facet in their Socialist agenda of controlling everything in our lives. However, everywhere this exists like England and Australia is a quagmire of increased gun violence by criminals who will always possess firearms because the the demand will always exist.
Only the threat of a self-defending armed citizenry can combat this quagmire.