The no-fly list is an important element of suppressing terrorism, foreign and domestic. It exists to prevent precisely this sort of situation - very few of these people were from within driving distance of Washington (or each other).
Whether it could do with more oversight is an independent question, with no shortage of people to ask it.
Terrorism is an over-applied label that is used to stoke FUD and expand executive power; yep I agree with that and I don’t use the term lightly.
Joining an organized movement to overturn a legal democratic election, breaking into the capitol, armed, and bludgeoning an officer to death is terrorism and a private company has every right to deny these people the privilege of flying on their airlines.