You're confusing that "Voltaire" quote (which wasn't actually said by him) with the American First Amendment.
The American First Amendment is predicated on America's cultural basis in the principle of free speech, which is embodied by the quote. The American First Amendment indeed applies only to government, restricting its overreach. But the principle of free speech is one of the core principles of American culture. It goes far deeper than the First Amendment.
If you don't want to debate with them then don't subscribe to their magazine, and leave them alone.
Nero