Pure speculation. I have given you one example (Hinckley) otherwise. Have you anything solid to rest your assumptions on?
The Secret Service will better protect the President if they aren't distracted by nutcases.
Either onlookers are considered a viable threat, or they are not. Once the decision has been made to allow onlookers to stay, all peaceful citizens must be allowed equal access regardless of their political views. Free speech and assembly, indeed our founding charter and our most core values demand it.
To deny an American his unalienable rights based solely on his political opinions is un-American and tyrannical. Our country was founded on civic duty and dissent. Want to call this a free country? Then walk the walk.
I stand by my statements. I have better things to do than to try and make a case to you.
Grow up.