A presidential power play in passive voice

It's a cliche of writing instruction to admonish students to prefer active voice verbs to passive ones.  There are obvious exceptions, such as when the writer wants to emphasize the object of the verb rather than the subject.

Normally, though, you'd think that active voice works better when you want to emphasize the agency of the subject. Not always!

Ever since the 2011 budget deal was struck at the literal 11th hour, Democrats have been bemused by the apparent perversity of the president taking credit for spending cuts that Democrats were presumably fighting tooth and nail to minimize.  As details emerged and the media pegged the value of the cuts at about half the original sticker price, the administration stepped up its bid to take credit both for minimizing the cuts (preserving key "investments") and for signing off on them -- boasting about "the largest annual spending cut in our history" (true only in absolute terms).

The outline of the president's long-term budget plan posted at WhiteHouse.gov extends this rhetorical act of taking ownership of the deal.  And, to return to grammar, the passive voice helps make the claim more emphatic:

The budget agreement negotiated by the President last week represented the largest one-year reduction in discretionary spending in our history, even as it invested in areas key to our long-run economic growth and competitiveness.

Why not write "The budget agreement that the President negotiated last week..."? Shouldn't a forceful president's activity be accorded the active voice? No.