Congress has the authority to do that, just like Congress doesn't have to pass individual acts determining each line item NOAA spends money on, or how exactly EPA will monitor industrial emissions.
I'm not saying Congress was smart to concede this authority to NSA and FISA; in fact, I think doing so was a dereliction of their duty. But what it wasn't was a power grab by the surveillance state. Rather, it was Congress being too short-sighted and bored of surveillance issue to properly regulate it.