Sponsors on Summoner's Rift

Everytime I look at "team banners" during LCS inside the bases hanging on the wall this thougth hits me: Why don't you let brands do this as commercial? I mean as a company having your logo on Summoner's Rift where millions of people hang out everyday must be the dream no?? I would say that's a win-win for everybody. I don't get why this hasn't happend yet, is there some rule that forbids this? What is the reason behind this not happening? I'm not very smart but I gotta ask..