The US has an obligation to spy to the maximum extent possible on foreign citizens and entities. Just as every foreign government has the same obligation to spy on the US.
Do we? I would say that the obligation of the US government is to follow the goals set in the Constitution as much as possible, until such time as the Constitution is amended or replaced. As much as spying does "provide for the common defence", it is defensible. When the spying alienates allies, it is too much. When the spying forces foreign-held companies to pull out, can it really be said to "promote the general Welfare"?
Why would there be such an obligation for unlimited spying?
And when spying becomes counterproductive, should we still increase the level of spying? When US-led coups destabilize a region, does this maximize national security? When US-based surveillance causes a drop in investment, does this maximize national prosperity?
It hasn't yet become counterproductive. And there is not a point at which becoming more aware of what everyone else is doing becomes counterproductive.
Spying allows increased national security at minimal cost compared to higher military preparedness. Thus, it allows the same level of security at lower levels of funding.
In addition, spying can give a leg up to domestic commercial interests over foreign commercial interests.
Why would there be such an obligation for unlimited spying?