I watched it live on CSPAN, have re-watched on my DVR. They specifically say it applies to US citizens right there on the floor of the Senate. The key wording in the bill is the use of "not required", when it comes to US citizens. Non-US citizens who are threats are required to be detained by the military, US citizens are not "prohibited", just not required to be detained by the military. This is why Paul and Durbin opposed it where as McLame, Graham, Levin and Kerry pushed for it.