I also found it surprising that it is apparently possible to modify client to teleport, move faster, and see data that is not allowed by standard client[1]. One would thought that after they went such a lengths to make it online only (making even map generation on server side) they would not trust a client. But apparently not.
[1] http://us.battle.net/d3/en/forum/topic/5978861022?page=1 "These programs may increase movement speed or teleport heroes from one place to another beyond what is allowed by game design. It also includes any programs that obtain information from the game that is not normally available to the regular player"