I can defend it, sadly.
I negotiate large, complex agreements for a living. I've been doing it for about thirteen years now. In that time, I have seen things similar to this on rare occasions: petulant or cagey parties who insist the deal isn't sweet enough, and that we have to make it better, without being willing to make a specific counter-offer. When we identify their kind, we walk away, and sometimes we insist on a new team if they want to come back to the table, after a stunt like that.
However, I don't ENTIRELY blame the Republicans for this. What someone is hoping for, with that kind of stunt, is that you'll negotiate against yourself. If you fall for it, they can identify what stuff you don't really care about that much (since you offered it up without even being asked specifically) and then they can take that and add to it the stuff they really do care about, in their counter-offer. They know you'll have no credibility if you then try to reel back some of your initial offer, in exchange for their new demands, because they already know that crap doesn't matter much to you, or you wouldn't have offered it. Maybe you'll even have offered something they thought was non-negotiable. You end up losing the little stuff and much of the big stuff, too.
It doesn't work against a skilled negotiating team, but the Republicans know from experience that it has worked against Obama in the past. Repeatedly. He regularly offers up concessions out of the blue, just like this. He offered up a higher Medicare retirement age and moving SS to chained CPI. He took single-payer and the public option off the table, for healthcare, in exchange for absolutely nothing. Why WOULDN'T they try this tactic again?
As long as Obama keeps falling for it, they should keep doing it. That's the defense: They're doing what has worked for them... forcing a truly horrible negotiator to try to anticipate what they'll like and come up with the first offer.
What Obama needs to do is to use his bully pulpit to call them out for it, demand they identify specific cuts if they'd like to see them, and stick to his guns. He desperately wants to negotiate with himself. Its in his character. Until he shows the Republicans he's mastered that bad instinct, they'll keep playing this game. They may be a pack of scoundrels, but they've got people on their team who know how to exploit Obama's weaknesses.