The president does not get to reserve an area for the public solely for his supporters simply on the virtue of his presence.
He can allow well behaved members of the public, or he can claim no one can be allowed because of security concerns.
The pres has plenty of private venues to reserve for his own supporters... the White House, any rented convention center, the possibilities are almost limitless.
But when he ventures out into the public and legitimate security concerns do not disallow onlookers, he is going to be subject to free speech. Why you might ask? Because he is the president of a free country, not the strongman of some pathetic banana republic.
Is that right ? Do you mean to say the Leader of the Free World isn't entitled to the same rights as a Hollywood film crew ? Just try nosing into one of their carefully selected location shoots with your protest signs and see how long you last.
And, what about a president's rights ? For instance the right of free association or a right to reasonable expectation of safety. But, of course, you understand all that.