They were arrested for his remark about serious consequences. So much for free speech. This is the kind of thing people were concerned about when the Patriot Act was passed. Using a law meant for one thing to punish people who don't behave in a manner consistant with the enforcer's beliefs.
Well, but threatening people has always been illegal. It didn't become illegal because of the Patriot Act.
Now just imagine how Hitlery Klintoon could use the Patriot Act if elected president. Speak out against eminent domain, which is a key component of the Communist Manifesto (which Hitlery and her ilk follow like a football playbook)? Violation of the Patriot Act. Speak out in favor of the Federal Marriage Amendment and against same-sex marriage? Violation of the Patriot Act. Say that global warming is a neo-lib-invented lie? Violation of the Patriot Act. Speak out in favor of border security and against illegal immigration and the North American Union? Violation of the Patriot Act.
Free speech doesn't cover making threats. Just like you can be arrested for saying "this is a stickup" at the bank, you can be arrested for threatening just about anyone. The flight attendant can't read his mind & determine if the nut is serious or not.