You may have heard of the “Countering Violent Extremism” program under DHS.
They’ve deployed the tactics used to fight terrorists against domestic citizens. For example, incels. When I posted on the web forum incels dot is, I didn’t know it was Department of Homeland Security operation against men who say they are romantically deprived. I didn’t know I was interacting with larping feds who encourage extremist rhetoric. Consequently I was put on a watchlist and am subject to intrusive monitoring.
I think it's a little fair to monitor groups that border on the edge of committing crimes to make sure they don't. If someone is going around saying they're going to kill people, even if they haven't, I'd keep a fucking eye on them.
Considering how much rhetoric incels and well known terrorist groups share, it doesn't seem unreasonable why they are included on terrorist watch lists.