I'm aware of those factors, but there's a balancing of legitimate interests. I disagree with your notion of active harm because the degree of self-censorship I engage is my choice.

Suppose I wish to immanentize the eschaton, and step one of this plan is the overthrow of the United States' government. Obviously the government as an entity has an interest in not being overthrown, so if I announce that goal and try to recruit people to it it's going to watch me increasingly closely in proportion to the degree of my activism.

Well, you say, doesn't that have a chilling effect? Maybe. it depends very much on the nature of my eschatology.

Perhaps I am a very peaceful person and my brilliant plan is to persuade everyone who wants to to meditate for 15 minutes a day at noon, thereby raising everyone's consciousness so effectively that everyone behaves angelically and the need for government evaporates. Unlikely, but fundamentally harmless. You're going to have a hard time finding any jury that will convict me for suggesting that people sit still and calm their thoughts for a few minutes every day, but only if they feel like it.

Suppose on the other hand that I'm a miserable old bastard and my plan is to build an army, storm every state capitol, kill 2/3 of the population, and then rule with an iron fist while bathing in the blood of virgins every day of the week, and that I have a cadre of trusted lieutenants and a vast trove of resources to get my project off the ground. Most people would prefer not to participate for reasons that I hope are obvious, and so operational security dictates that I be very covert indeed.

Obviously, the bulk of political activity takes place in the center between these similarly unlikely extremes. Equally obviously, extremism exists and some of that extremism manifests as threats rather than opportunities. It is, inarguably, the government's job to protect people from reasonably foreseeable threats, and gathering information on the environment is an unavoidable necessity in the pursuit of that end.

I don't feel my liberty to be compromised by this, primarily because I have no plans to extralegally reduce anyone else's liberties. Is it possible that corrupt individuals, factions, or whole institutions within government might recognize that I am a peaceable person, but decide to make my life miserable anyway? Yes, of course it is.

But that is a risk I must accept as it is a basic problem of human relations. My personal philosophy on such matters is basically a Stoic one, that I am primarily accountable to myself, and even if my liberty or life should be threatened by others, what maters above all else is my relationship with my own conscience. There is no way to legislate moral goodness; people can observe the letter of the law but still have evil intentions towards others and even act upon those (indeed I'd say that describes how a large part of the USA functions right now).

Adding more laws to constrain government from doing things that might be used for evil is not the answer. First, such laws are easily circumvented. Second, laws are easily suspended, by fiat or by consensus, in times of emergency, and the belief that they won't be is facile. Third, such laws would impinge upon legitimate actions taken in pursuit of legitimate objectives, ie the detection of threats. Fourth, given the existence of other powerful nations with powerful intelligence-gathering capabilities, for a country's government to put itself at an informational disadvantage relative to others is to put the citizenry at a strategic disadvantage, which is to endanger them, which irresponsible given government's obligation to protect.

It's foolish to only consider one side of an argument or to only prosecute one's own interests. Natural, but foolish. I think policing is badly corrupted in many jurisdictions, but a policing power is an essential role of government that has been instituted because it works better than the alternative. I loathe war and militarism, but a country that dismantles its armed forces in the name of peace makes itself vulnerable to invasion by less altruistic nations, to the detriment of its residents. I don't enjoy fighting, but I am good at it and will defend myself if someone attacks me rather than allowing them to use me as a punching bag. Likewise, I come to the aid of others if I happen upon a situation where I see another being abused.

No offense, but I think the notion that governments should refrain from any sort of pre-emptive intelligence collection is ridiculous, and I would no more support it than I would walk around an unfamiliar neighborhood with blinkers on to avoid seeing anything that might upset me. I feel that your and others' insistence that government abstain from any sort of intelligence gathering is equivalent to trying to solve health problems through prayer, or trying to prevent unwanted pregnancy and venereal disease by telling people not to have sex. It's neither practical nor sensible. Just as adulthood involves making complex and often unpleasant decisions about one's person and relations with other persons, citizenship requires making complex and difficult decisions about the pragmatic balance between orthogonal legitimate interests.