(Devil's advocate mode) Do you suppose that such incidents may be deliberate provocations, part of a campaign by anti-Bush activists to make America look like an ugly fascist state and damage the Bush administration's already shaky reputation?
Another devil's advocate comment: Malls are indeed private property. While I can see the hypocrisy and stupidity of a store in the mall selling the shirt and then the security guards at the same mall finding offense with it, they are able to have someone removed from the premises if they see fit. Complaining about free speech on private property is a bit useless. Once the request was made, the police would have had to comply.
That said, I think the security guard involved is completely over-involved in his/her job. I can't stand flag-waving, "America: Love it or leave it" morons. Mall personnel should have a serious talk with that security company about guards interfering with the customers.
http://www.metafilter.com/comments.mefi/24061
Some of the background to this story is coming out at Mefi; there was a protest their earlier in the week; the mall security are on a hair trigger and jumped the gun. Not quite as bizarre as the guy locked up for protesting about his pregnant wife being molested by airport 'security', but, disturbing nonetheless eh?