That's strange to me too. People on the terrorist watch list can't fly but they can buy guns. That doesn't really make sense.
On the one hand, people who have been found to be a legitimate danger of terrorism by a reasonable process including due process of law arguably shouldn't be able to buy guns. Or fly on commercial airliners.
OTOH, I can also see a reasonable argument to be made that the various terrorist watch lists (including the no-fly list) do not meet the appropriate standard of due process for either or both of those consequences.
All of those things have been proposed this year or are already enacted.