Glojo,
It's not really Obama's decision- the Pentagon (Obama) can ask Congress to change Defense funding to cancel/delay/suspend the F35B. It's then up to Congress. They often disagree with the Pentagon- V22, SR 71 and F136 engine are past examples where Congress funded something the Pentagon wasn't ( at the time) keen on.
It boils down to pork-barrel politics and how much political influence the USMC (and UK) can deploy in that process. The USMC have big clout, and if LM are smart they have done as Bell-Boeing did with the V22 and make lots of F35B peculiar components in lots of different states, thus assuring the support of senators in those states.
Nonetheless the fiscal cliff approaches and Congress has either to re-visit US spending and borrowing, or find and approve some serious spending cuts.
N