I'm sick of the same arguments about this stuff, so I'm just going to say this, "legal" and "constitutional" are not synonyms, so I wish that the government employees would stop using them hand in hand. Just because there is a law saying you can do it, does not make it Constitutional.
Legal is a subset of Constitutional, since a purported law saying permitting something prohibited by the Constitution is no law.
There's also a fairly decent foundation in the case law (indeed, its part of the reason FISA was passed to constrain foreign intelligence surveillance) that warrantless surveillance authorized by the executive for foreign intelligence purposes (including surveillance of Americans for that purpose) is Constitutional and legal in the absence of Congressional action.
A bigger problem that conflating "legal" with "Constitutional" is conflating either with "desirable". Just because the laws under the Constitution, or the Constitution itself, allows something doesn't mean that it is good to do.