Last time I checked, most web services (or even telephone providers) are not keeping track of or verifying nationalities. Thus, any blanket surveillance means they are inadvertently spying on US citizens/persons.
Let's say you have an American, living abroad. He signs up for a Skype account. Skype does not verify nationality or identity. He starts using Skype to call phones in some country the NSA finds suspicious (say, Afghanistan). A dragnet which would intercept these communications automatically would be illegal, as one of the parties was a US Citizen... and they would need a warrant.
The whole thing is ridiculous. The whole question of "do I need a warrant or not" requires you to prove that you do not. But to whom? You are in the pre-warrant phase... Crazy.
Let's say you have an American, living abroad. He signs up for a Skype account. Skype does not verify nationality or identity. He starts using Skype to call phones in some country the NSA finds suspicious (say, Afghanistan). A dragnet which would intercept these communications automatically would be illegal, as one of the parties was a US Citizen... and they would need a warrant.
The whole thing is ridiculous. The whole question of "do I need a warrant or not" requires you to prove that you do not. But to whom? You are in the pre-warrant phase... Crazy.