Why should the government preemptively keep tabs on anyone? Should the EPA and HUD be able to place sleeper agents in right wing community organizations to monitor them? Should pro-secessionist historians have their phones bugged and be trailed because they don't believe in the legitimacy of the federal state?

Aside from that general principle, there's also the reality of how its implemented. Most of the time sleeper agents are used to get evidence for trumped up, meaningless charges like "interstate travel with intent to riot." That's the best case scenario. At worst, these agents, on not finding something of even minor note, will attempt to incite violence to prove that "those anarchists have really been violent all along." See, for instance, Brandon Darby.

Edited to add: I do, for the record, think that if there's actual evidence that someone's trying to blow up a bridge, monitoring is appropriate (though I don't see why simply arresting them wouldn't usually be sufficient if that were the case). But usually these things are more fishing expeditions than anything, against people whose ideology the State finds distasteful.