While it goes without saying, Blizzard is saying it out loud so that Diablo III players understand the ramifications of cheating or using exploits: if you do, then you will be swiftly and permanently banned. Any users found using hacks, bots, or modifications of any kind will be permanently banned from Battle.net.
"Playing Diablo III legitimately means playing with an unaltered game client. Doing otherwise violates our policies for Battle.net and Diablo III, and it goes against the spirit of fair play that all of our games are based on," the company wrote on Battle.net. "We strongly recommend that you avoid using any hacks, cheats, bots, or exploits. Suspensions and bans of players that have used or start using cheats and hacks will begin in the near future."
Anyone that is shocked or dismayed about being permanently banned for cheating is either in denial or grossly ignorant of how Blizzard operates all of its online-enabled games.