You left out a pretty important detail. Your "insurgents" in America aren't shooting people or planting IEDs. Communicating and protesting, on the other hand, are sacrosanct rights in the US.
The FBI has a long history of attempting to infiltrate and destabilize these groups. In the early 2010s there was a push to infiltrate right leaning groups. They especially called out in their published documents disgruntled veterans returning from the wars and unhappy with leadership noting a worry they would use the skills picked up at war at home.
It's absolutely no surprise that the FBI would investigate this behavior.