Don't know why you say this. I haven't seen any scenario in which Obama is the sole decider of what gets cut. Far from it.
The ONLY alternative way to prioritize is Congress. Congress can ONLY force something to be spent if they can muster enough votes to override a veto.
If money is not available, the Executive Branch is tasked with prioritizing how all money is spent. The assumption is that Obama would do his best to prioritize wisely.
Absent a raise in the debt ceiling, we are looking at immediate spending cuts of 41% or more.
Do YOU want to give Obama the power to decide how to make those cuts? Do YOU want to give him that much power?
(Yes, I know. We CAN'T go on the way we are either. Houston, we have a problem.)