It's THEIR convention, not that of the Greens, the Libertarians, the Republicans, the Nazis, the Facists, the Communists, the Socialist Workers' Party, or anybody else. It's the convention for the Democrats.

As such, it's like a private party: if you aren't invited, you can't come. If you crash it, you can be arrested.

You have every right to speak your mind peaceably, to peacefully assemble, to petition the government for redress, etc.

But it's still the Democratic National Convention, and as such the Democratic Party can and does have the final say on how it's run and who gets to speak.

Live with it, or organize your own convention.