It looks to be the handiwork of Murphy's Bleachers on Sheffield, located across the street from Wrigley Field.

Update 6:39 p.m.: The inevitable apology, from owner Beth Murphy:

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"Some guys thought it would be clever," Murphy told the Tribune. "Obviously we are ashamed. I'm so upet. We put up (a sign with) our sincerest apologies. My father was a veteran, and my uncles. This was just not us. Personally this is devastating to me. I apologize to everyone who was offended."