In a sense, they are like children with an exciting toy. A child loves his toy when he gets to play with it, much like supporters of free speech love the idea when it means they get to say whatever they want. All too often, however, these same people balk when others begin to talk back, particularly if what the others say isn't entirely positive.
That's when they show their true colors. That's when, in their ever-increasing fear of criticism, they go on the attack. They threaten, they censor, they do whatever they can to make the other person shut up. They actively seek to deny the other party the same freedom of speech that they themselves enjoy. Much like the child with the toy, who throws a tantrum when he is asked to share.