I completely agree. There are two important points here. First, they don't currently know the answer to the question, so they would have to figure it out, probably by examining a bunch of the conversations to identify who is and who is not a US Citizen. That examination would mean violating the privacy of US Citizens. (Collection does not equal examination.) Second, the wording in the article is a bit misleading. The article says that the NSA "won't" give this information to the Senators. Technically, that is true, but it would probably be more accurate (and less inflammatory) to say that they "can't" provide the information. Won't and Can't imply very different intentions. I'm not saying it is right/acceptable that this info is not known, just saying that the way the article is written is a bit misleading.
How about this slight modification: "SOPA is the equivalent of allowing the US Government to shut down a restaurant because Disney thinks a customer may have drawn Mickey Mouse on a napkin."