It is a stupid self-destructive ideology, but not terrorist. If they were trying to impose that ideology through the random killing of large numbers of innocents in the hope that, out of sheer terror, we would cave in to their wishes, then they'd be terrorists.
Instead, they are simply criminals. Some of them violent.
We are all indeed sovereign. We can choose to submit to the society in which we live, try to select another one, or live by our own rules. Each choice has consequences.
Considering that, with all its failings, our society in the US is the best in the world, and not so far removed from the best it ever was that we cannot once again achieve that level of greatness, I choose to submit to this society that respects the individual more than any other on Earth. Almost all of us, for our own reasons, also remain a voluntary member of this collective. The consequence is that we often have to unjustly submit to the society's failings. The upside is that we are freer to do our will than we would be in total anarchy, where our neighbors would be able to take our freedom if they have the physical power to do so.
The consequence of not submitting to the society that almost all of us accept is that one will be a criminal and will likely end up dead or in jail. Stupid. The upside is that he can do whatever is in his physical power to do (in the case of the father and son introduced in the story, not much) until society decides to stop him.
Anyway, the real meaning of our individual sovereignty is that we, as a society, formed the government to whose rules we voluntarily submit; that we, as a people, are the ultimate source of the authority of the government. We, as true sovereigns, are free to change that government when enough of us are willing to pledge our lives, our fortunes, and our sacred honor when, in the course of human events, it becomes necessary.
When we do it in a unified way, to secure the rights of all, it is a righteous revolution. When individuals do it in a way that even those who love Liberty think is criminal, then they are...well...criminals.