For those of you unfamiliar with football, the term "12th man" refers to fans in the stands who make as much noise as possible to disrupt visiting offensive units (e.g., quarterback play calling and audibles) and Kansas City's Arrowhead Stadium is notoriously hard to play at for that exact reason.
The Kansas City Chiefs have a "Fan Code of Conduct," which is essentially a list of rules that fans have to follow when they're attending a game at Arrowhead. In theory, it's not a bad idea, as you'd like your stadium to provide a friendly atmosphere for families and people who aren't drunken hooligans.In practice, though, it might not be such a great idea, especially if the person responsible for coming up with the rules is an 85-year-old woman who teaches the 2nd grade and regards standing up as one of the evil things that young whippersnappers often do. Seriously, standing erect is the No. 2 item on the list of things that are prohibited.
Making fans sit down is anathema to the NFL and professional sporting events generally.