They consider it ambush marketing. The reason is that they don't want companies that have not payed for sponsorships to get free advertising. The Olympics and other sporting events have similar rules. In this case, South African law applies: " the Contravention of Merchandise Marks Act."
FIFA ejected 36 female Netherlands fans from their World Cup match with Denmark for wearing an orange mini-dress designed by a beer company.
The dresses were made by Bavaria beer and despite the outfits containing no branding, the organizers said it was against their rules on "ambush marketing". Read on at Yahoo! World of Sport.