"Disruption" is the key operative here and not whether or not someone's free speech is violated on private property.
In a Milwaukee suburb there were problems at a mall where patrons were being harrased by teenagers. On at least a couple of occasions fights broke out amongst a large group of teens which threatened to get very ugly.
I don't know what the resulting action was since I have not heard any follow-up on the story. But one solution was to institute a curfew on teens after 6pm.
As for this person's right to wear the t-shirt that he had on, to imply that a shopping mall is the ONLY place where political speech is possible is rather narrow. There are plenty of public places he could have used such as a sidewalk, on a road, or in a public park. If I saw him in a mall I would ignore him unless he were to come up to me and spouted forth any of his diatribe.
The public square is not dead; you just have to find someplace PUBLIC to move it to.
I have no problem with cops arresting people who are causing a disturbance or with teen curfews in the public square or at malls.
I do disagree with you on this statement, however.
There are plenty of public places he could have used such as a sidewalk, on a road, or in a public park.
Sidewalks? Many towns don't have sidewalks. Roads? That's suicide. Parks? Okay, but many parks are deserted, so if your purpose is to communicate a political message, you'll be giving it to the squirels.