For the millionth time, why didn't they get an effing subpoena after the fact.
There's nothing in 18 USC 2511(2)(a)(ii)(B) requiring a subpoena. It says the exact opposite. It says the Executive can tell the telco that no warrant is needed.
It's a recognition that when Congress created FISA, it could not anticipate every possible surveillance requirement in the future. That its law could not render the Executive powerless in such circumstances.
I agree it's not a desirable situation to be in. It leads to a constitutional crisis. An ambiguity of co-equal powers. Fortunately, it was resolved with an expansion of FISA and the President folding into that expansion.