The reason the NSA hasn't gone after anyone else is that it would probably be illegal to -- not that the program in general is legal, but it would be more blata

The reason the NSA hasn't gone after anyone else is that it would probably be illegal to -- not that the program in general is legal, but it would be more blatantly illegal than simply focusing on groups affiliated with al-Qaeda. The AUMF and constitutional case law give the executive branch a fair amount of discretion when it comes to national security. It's pretty hard to shoehorn drug cartels, bankers, and your run-of-the-mill murderers under national security. As such, prosecuting those folks using NSA-gathered evidence would force the NSA to reveal more about the program than it's comfortable doing.