I go to basketball games and they do not allow people to stand around holding a beer on the concourse. They can do it inside by the refreshment stands but not in the arena area.
Basketball is a bad example. Fire doors separate the concession area from the arena at XL and you can't see anything from the concession areas at Gampel (where they don't sell beer)...Technically, a person is not in the viewing area until they step over the red line at Rentschler. They are still in the concession area. People are cleared from the concourses at Rentschler just as they are the XL and Gampel, The sidewalk behind the lower bowl is continuously cleared of loiterers by police and security. The difference is that Rentschler is (correctly) designed to see at least part of the field from the concession area.
The simple answer is that the ticket = assigned seat = must occupy assigned seat is only enforceable within reason (i.e. checking tickets in the chairback rows of the lower bowl in the first half). Giving a powertripping security guard even more authority takes away from the stadium experience and people will actually leave because of it. Stadium management can't help if the product drives people away, However they can affect their own employees/contractors from doing so. The longer a patron is in the stadium, the more likely they are to purchase refreshments = more revenue. It's like when they say, "if you don't notice the ref, he's doing a good job." If Security is maintaining reasonable order without interfering with the fans' experience, live and let live.