The phrase is "reasonably persuasive answer"
Is there a Second Amendment right to bring a loaded firearm to a place where the President of the United States is in your line of fire?
Apparently, the Secret Service enforces a weapons ban, and the area around a President protected by the Secret Service is considered Federal territory. But it's hard to see how, if there's a Constitutional right to carry weapons, that right can constitutionally be abrogated just because the President happens to be around.
The "reasonably persuasive answer" is
The President isn't in a gun-free zone; he's in a zone where there are lots of people with guns, all of whom are loyal to him.
That's the key point; most of us, most of the time, don't get that as a possibility. So the rights line is a second-best, as most protected rights are. Since we can't be assured that there will be people with guns who are on our side all the time, we get to carry a gun.
Professor Kleiman's correspondent has presented a reasonably convincing argument for the constitutionality of the law which forbids the President of the USA to bear fire arms. Fine no problem, he has secret service agents so he doesn't have the right to bear arms (also he has a press secretary so he can be impeached and sent to jail if he dares to speak in spite of the first amendment, plus he gets to stay in the White House for free so soldiers can be quartered in the White House whether he wants it or not in spite of the third amendment).
The question was why I can't carry a gun anywhere near the president, not if it could be ok to forbid Mr Obama from bearing arms (at least if your interpretation of the constitution is not in any way constrained by, you know, the text).
I wonder what Prof Kleiman would consider to be an unpersuasive answer.