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:
"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."