Observing and recording law enforcement is generally not illegal, however threatening it might feel to them. It's unfair to compare that to an event where protesters broke into a government building, damaged property, assaulted law enforcement, and made violent threats against elected officials. As to the generosity of letting people know they might be added to an official list, it seems more calculated to intimidate than to inform.
If DHS has probable cause that they've violently attacked an officer why aren't they bringing that to a judge and jury rather than some secret list?
Being put on a terror watch list doesn't mean you've assaulted an officer, it means they have reason to believe you're a risk of committing an act of terrorism, which is a reasonable concern for DHS agents. Assuming the man in the article isn't a literal terrorist, LARP'ing as a terrorist is dumb and it doesn't seem unreasonable that he'd be at risk of getting put on a watch list.