In that pre-digital era, the spying consisted of listening to telephone calls, opening mail, and using undercover agents to infiltrate political organizations and, as we know, break into their offices. Nixon claimed he did this for the protection of national security. He also claimed he was entitled to break the law and violate the Constitution. "If the president does it, that means that it’s not illegal," he once famously said.
Since no one was prosecuted on the basis of data stolen or retrieved by his spies, the courts rarely encountered this behavior and never had to rule on it, and thus it went largely unchecked.