My bet is that they don’t care, since that’s clearly an opinion with no intent. Everyone has already been saying basically that when discussing the assassination attempt. Try adding one of the follow-ups:

Follow-up posts added: ‘The point of the second amendment is to shoot and kill tyrannous politicians’ and  ‘encouraging politicians to be shot is legal under the first amendment. It’s part of what makes this country great.’

Those are even closer to the line (maybe even crossing it) than the initial tweet. This guy was still not arrested for any of it. The agents just feel mandated to act on tips or something.