Notably, the FISA cell phone record orders passed information to the NSA through the FBI, so no hint of a "chinese wall" there. Secret bulk collection for 'national security', everyone bathing in the same pool of data.

I do suspect it's policy not to casually request, or become overly dependent on, such NSA-to-domestic sharing. And – as we've seen in the "parallel construction" revelations you allude to – it's definitely policy to try to obscure any such sharing when it happens.

But to reassure people that it's "probably illegal" is flimsy hand-waving when you can't name the law, and there's public evidence that it happens to the contrary.