"terrorist" is used, and can be used, to define a great deal of people. Say, those protesters are speaking out against the government and raising potentially bad points? Well, they're terrorizing US citizens, and can be detained indefinitely.
The military has never had authority on US soil, except for US military bases. The Uniform Code of Military Justice (UCMJ) should only apply to soldiers, not private citizens. Hell, if we go by the UCMJ, no one is allowed to speak against a government official.
Freedom of speech is one of the most important rights in this country, even if people say stupid, bigoted, even hateful words. This indirectly allows the US government to control speech of US citizens by declaring the United States a war zone.
No, the wording is very clear on the 'covered persons' section. It says persons
determined to be a member of Al-Quada or the Taliban or
significantly aided them. It doesn't say "suspected" or even "terrorist" or "extremist" anywhere in there. It doesn't say anything about sympathizers or anything. Also, the National Guard has always been able to arrest people, they are given arrest powers anytime their state govenor says so for emergencies and civil unrest and insurrections and whatnot. Also, US citizens cannot be held in military detention or be subject to any sort of military justice unless they are in fact a servicemember.
This law really doesn't apply to protestors shouting vaugely seditious things, socialist book clubs, Wahhabist knitting circles, or even run-of-the-mill extremist groups like private militias and people from Montana. Other acts prior to this one already cover them to some extent, though they require warrants and justifible causes to go after groups like these.
The only mechanism for abuse I see when the President or Secretary of defense waives the rights of an individual US-citizen terrorist type and goes and arrests or kills them, like they did with Anwar al-Awlaki. Although frankly, Anwar al-Awlaki probably got what he deserved.
Basically, all this bill does is officially codify in law, what the business as usual doctrine has been when dealing with Al-quada terrorists and unlawful combatants. Sending them to Gitmo indefinately and whatnot. It really isn't that threatening unless you are plotting to bomb something with your Al-Quada friends, in which case its only as bad as the unofficial doctrines before the bill anyways.