Umm, huge difference between a law enforcement office joking about killing someone while on duty and any civilian joking about something non-lethal. Law enforcement agents needs to be held to a higher standard. Period.
In the case of this guy, saying something like this shouldn't be a crime. No one panicked. No one was hurt. Nothing was destroyed. Nothing happened. Had he gone through with it, that constitutes a crime.
When something can't be said out loud, we can no longer have a reasonable discussion about it. Like in this case, he had been trying to expose security flaws in the airline system for some time and was told to stop. Instead of having an open discussion about it and allowing fixes to be put in place. This just hid the problem.
The only time words should be criminalized is when it causes someone harm and the intent was to do so, ie yelling fire in a crowded area, threatening to hurt someone. I don't think his comments fall into that category.