In his Hamdan dissent, Justice Scalia says:
Of course in its discussion of legislative history the Court wholly ignores the President's signing statement, which explicitly set forth his understanding that the DTA ousted jurisdiction over pending cases.Dear Justice Scalia: they call it legislative history because it's the history of what the legislative branch does on its way to passing a law.
The President is the Chief Executive. That means he's part of the Executive Branch, the branch that executes the laws that the legislative branch enacts.
I know this will come as a surprise to you, but the executive branch isn't part of the legislative branch, and no action by the executive branch is legislative activity. Specifically, a signing statement by the Chief Executive cannot be 'ignored' by the Court in "its discussion of legislative history" because the signing statement is not part of the legislative history.
If such a thing as the 'executive history' of a bill were to be defined, Bush's signing statement would be part of that history. But so far, the judicial branch (the branch you're in, remember?) hasn't defined such a thing. And it certainly hasn't explained what part of the Constitution would ascribe any weight to such a statement to begin with.