I guess that depends, do you think that an administration would be willing to prosecute its predecessors for doing it if it were against the law? Maybe you don't because of mutually assured destruction, but lately there's been a lot more appetite for prosecuting people in the other party (with some charges being extremely trumped up, if you'll pardon the pun), so I don't know that this is a given anymore.

Even if it were perfectly illegal plain as day, they’d launder the intelligence gathering through a partner intelligence org like mossad.