NSA isn't allowed to spy on US citizens. NSA is a US military organization under Department of Defense, and Posse Comitatus act makes it unlawful for the US military to act as a police force in the US.
One of the few good things revealed by Edward Snowdens leaks was the fact that the NSA has filters for intercepted communications to filter out comms from US citizens. This was in top-secret programs that had no reason to be publicly known, and yet the NSA still had these filters installed anyways, because everyone in the NSA understands that they're not a law-enforcement agency, because of Posse Comitatus.
> Posse Comitatus act makes it unlawful for the US military to act as a police force in the US.
Strictly speaking, that's not correct. The Posse Comitatus Act just changes the status of using the military as a police force from “allowed because any person or group can be deputized as a police at any time”, to “the US military can be used as a police force only under the laws specifically allowing and governing the US military as a police force.”
(Of course, the Posse Comitatus Act is a criminal law, which means in practice the primary mechanism for enforcing it is for the executive branch to arrest and prosecute offenders. This works tolerably well to prevent, say, a rogue sheriff calling up his buddy who happens to command an infantry company to come help out, but not particularly well to dissuade the President from directing the military for policing as a matter of Administration policy.)
In principal the courts can constrain the government based on it, as well, but it is noteworthy that the determination that the deployment was illegal in the case filed by the State of California almost immediately when courts were open after the initial LA deployment was announced on June 7 and before troops arrived on June 10 was just released, on September 2, nearly 3 months later. And is on hold for 10 days to give the government time to appeal. So, one might consider the courts to not be a meaningful constraint, here.
So what would you say about the PRISM and Upstream programs where metadata about millions of Americans was collected? Doesn't it seem as if they could target any US citizen by just pretending to target any foreigner they communicate with?
I mean that the US government could have laundered some of the tools it is not supposed to have developed against US citizens through Israeli companies. (We don’t have any evidence of this in this case)
One of the few good things revealed by Edward Snowdens leaks was the fact that the NSA has filters for intercepted communications to filter out comms from US citizens. This was in top-secret programs that had no reason to be publicly known, and yet the NSA still had these filters installed anyways, because everyone in the NSA understands that they're not a law-enforcement agency, because of Posse Comitatus.