Here is a Liberal's (me) perspective:
The mall had the right to tell them to leave and then have him arrested when he wouldn't, but they (the mall) were morons for provoking this in the first place. The shirts did not have any inflamatory, or even specific, slogans. No "Bush blew up the towers" or "No war in Iraq", or any other BS. They simply had the old slogans "Give peace a chance". I see t-shirts with all sorts of slogans, some prettyy crude, in the malls in my area. The statement was hardly inflamatory enough to warrant the reaction that the mall managment had.
Of course it backfired on them as they had 100 active protesters there today.