The NSA doesn't need warrants because breaking federal law is easier to get away with when you have one of secrecy, some relation to security, presence in the meetings where you'd be discussed, and largeness (which provokes acceptance), and they have all four.
Israel doesn't need warrants because it's not an American government agency conducting homeland security.
Those are the forces at play, but you’re just guessing how that manifests itself in the real world. I’ll bet it’s all a lot more bureaucratic than you think it is— secret FISA courts wouldn’t exist if it was just a free-for-all the second it was out of sight.
That’s why these agencies buy data from brokers: sidestepping the mechanisms in place to stop them from getting it straight from the source for free.