Earlier quoted context omitted.
If he was going to harm the aircraft, he wouldn't have announced it in a tweet. If the FBI already had reasonable suspicion he would, they shouldn't have let him get on the plane in the first place. There is no rational reason for their action. This is simply a PR move by United and the FBI. Which leads to the same conclusion: just don't tweet things like this. Not because you're wrong, but because they are assholes.
"If he was going to harm the aircraft, he wouldn't have announced it in a tweet." Many, Many bad actors have a pretty good trail/history of signals that made it clear that they were going to do something stupid. And a lot of them are stopped because authorities stepped in when those signals were reported. Would you want to be the person who was notified that someone was communicating they were considering interfering…
That's the problem with concentrating blame. It leads to net counterproductive behavior. One in such a position is thinking about their position, not just the people they are protecting.