I'm not sure if "allow" is the right word here. Really the NSA can set up another Room 641A at any company they want regardless of that company's wishes and with a national security letter the company can be prevented from even telling anyone (like their customers or the media) about it. I was furious when Obama granted telecom companies immunity from prosecution for their role in the warrantless spying, but really it's not exactly fair to pushing people who are strong-armed into compliance.