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.
First, in the active voice, "negotiate" is usually transitive: you negotiate with somebody, which means sharing the credit. That problem is solved here by having "the President" serve as object of the preposition, hermetically sealing the negotiating process and putting all other negotiating parties deep in the background. Second, I suspect that "by" triggers a "byline response" in readers: this negotiation was "by the President," as a book or article (or government) might be. Finally, and somewhat paradoxically, "negotiation" is a process the president wants to be associated with, and letting "negotiated" immediately follow "budget agreement" emphasizes that process. "Negotiated" immediately modifies "budget agreement," and "the President" (alone) modifies "negotiated." Finally, there's the old Latinate "sting in the tail": the most important thing in the string comes last (never mind the throwaway "last week" that rounds out the phrase).
The public, Obama has noted repeatedly, wants compromise. Here we have a grammatical bid to have that cake and eat it: the President negotiates deals that balance the two parties' competing needs while rendering relatively invisible the others at the table. In other places, he will credit "Democrats and Republicans" who "come together," but here in his plan, his agency is the (passively made) point.
Afterthought: insofar as the editorial decision in question here was conscious or semi-conscious, my guess would be that the motive for using the passive voice was simply to avoid inserting an active voice subordinate clause in a long predicate phrase. Doing so forces the reader to sort of mentally start over in mid-stream. "The budget agreement that the President negotiated last week" is harder to roll with than "The budget agreement negotiated by the President last week."
As for the broader negotiating strategy, or rather style, please see Obama: better hedge than wedge.