So why are Wrigley Field ushers so tough on fans who move seats?
The only rules I could find on the Cubs Web site related to fans and seating are from the "Guest Conduct" section of the "Ballpark A to Z Guide," also called the "Wrigley Field Fan Guide -- 2008":
"Customer Service is a major objective of the Chicago Cubs. The team strives to provide its Guests with the most positive baseball experience in the Major Leagues in a safe, comfortable environment. To help us achieve our goal of outstanding customer service, we ask for fan cooperation in the following areas:
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I had an experience similar to the folks ref'd in your original post. A couple years ago I was at a game and able to move up to good seats several rows back of the Cubs dugout. One guy sitting three seats in told my companion and I it was cool to sit there, as they were his seats, and his guests had to leave the game early and would not be returning. It was top of the 8th, as I recall, and the Cubs' reliever was struggling badly. I should mention that I had my Sox cap on, and was razzing the pitcher a bit. Nothing obscene, I swear, and tame by almost any general ballpark standard. All of a sudden, one of the shrivelled old lady ushers appears and asks for my ticket. I showed her my actual ticket, while our erstwhile "host" showed his proper ducat and explained that they were his seats, etc. No dice, said the usher, glaring at my White Sox cap. She threatened to summon the cops if I did not move, posthaste. I thought at the time I was being singled out only because I was a Sox fan. Glad to see the ushers at Wrigley are equal opportunity bigots, however, hating the Japanese as much as they hate south siders. I feel so much better.