You can't say your going to shoot someone, whether you are joking or not. Mr Brown is learning a valuable lesson.
Perhaps he will get lucky and have this felony charge dismissed. My gues is that he might but not without learning that political dissent is open and welcome in America - but jokes about assassination attempts are not.
I recommend that all of the other posters on this board that believe they can "joke" about shooting someone, and specifically a presidential candidate, to provide their full name, date of birth, address, and telephone number in a post here on this site and make the same "joke".
Let's just see what happens, future felons.