Are you sure there weren't TIFs involved in building the mall? The mall here
in Oshkosh (now defunct, turned into offices) was build with city money, the
newest upscale condo being built downtown is mostly TIF money, likewise the
newest big "low rent" housing development.
There's worse state involvement than that - an appalling number of malls get eminent domain support from towns to force nearby landowners to sell them the land. Costco's management recently rejected a shareholder proposal that would have forbidden Costco to use eminent domain when building new stores.
But even without that, most malls are owned by corporations, which only exist because the State calls them into existence, and in return for that favor it's legitimate for the state to place arbitrary restrictions on what they can do. (That's a political assertion, not a legal assertion - from a legal standpoint, the Pruneyard decision probably supports the guys with the shirts.) Malls that are owned by private individuals or partnerships ought to be a different case, and apparently there have been some courts which have decided that Pruneyard applies to malls with "public" walkways outside the stores, but doesn't apply to the insides of big-box stores.
The guys with the shirts were interviewed on several TV shows last night -
apparently the guards approached them while they were eating in the food court,
and started off by demanding that they take off the shirts or else leave.
The guys with the shirts may have just been abbreviating their descriptions,
but they appear to have forgotten the magic words for this sort of situation,
which are "Get your manager" and optionally "Who's the manager from the
mall company?" (since mall rent-a-cops are often from a rent-a-cop agency
rather than direct mall employees.)
One thing that came out on The O'Reilly show was that, while the rent-a-cops'
behavior seems bizarre and jingoistic, apparently there's some context to it -
a couple months ago, there was a group of people who did an antiwar protest inside the mall,
carrying picket signs and yelling a lot, so the guards may have assumed that
these guys were part of the same thing.
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Later updates - On Wednesday, about 100 people did a protest march at the mall
protesting the arrest. Mall management has asked that police drop charges.
