Lets take a step away from the hard and fast rules for a second. The issue here for me is that the ads will go where the players are running and playing, not just surrounding the space where they play.
Now to the tradition question. All the changes to "tradition" people have mentioned in this thread have been made in order to improve the game or its popularity (eg. lowering the mound, the DH, opening day in Japan, juicing the balls
), which is far different than uglifiying (if thats a word
) the field just to make more money. The placing of ads on the field does not improve the game or increase its popularity, it simply makes more money for owners and the league.
Nobody likes the ads everywhere in ballbarks, but since they are not seen with the players, they are not as bad as ads undernieth a player's feet.