Yeah, easy to say, impossible to do. When you go offline you can do EVERYTHING with the data, including the deletion of any flag that says that character was played offline.
So it would be like this:
Blizzard: Make characters played in offline mode ineligible for PvP and any other forms of competitive play in the future
Hackers: Patch the game so that Blizzard would never know that a character was played offline
Companies have probably put billions of dollars in offline security systems and DRM and some other shit and they were all pretty much avoided. So no wonder they went online with this shit, the only thing I wonder is what took them so long. But even so, most software that require an internet activation are still cracked and patched.
So the logical conclusion for online play is indeed "do anything through our servers", in that way you can have much more control. Sure there are exploits from time to time, but "some dupes" is nowhere near "a fuckload of dupes and cheats" which happens offline