Having a drink next to your register looks unprofessional. It also does pose a spill hazzard. The largest reason of all is that in some areas of the country, health codes prevent anyone from having food or beverage other that a bottle of water in ANY work area. Wal-Mart finds it easier to make policies that are the same nation wide rather than only for certain states. It's easier to enforce a policy if everyone plays by the same rules.