Pheace: I wouldn't put this solely on the desire to DRM the user. It's also marketing. Making sure you can use the single player characters for multiplayer makes the step-in that much smaller and makes sure a higher percentage of players participates in that part of the game. It also helps create social bonds, peer pressure, viral marketing and all those things to a small extent which is one of the main reasons WOW has been around so long.
Yeah because Diablo 3 is such a barely know new IP and such a risky venture that they have to resort to such dirty tricks to get peoples to buy it. Do they really have that little confidence in the ability of their game to sell normally otherwise.
Also unlike WoW there isn't a monthly fee (at least not yet) so it's not like they needs peoples to continue playing it, unless of course they expect the money they will get from the auction house to make up for the lack of monthly fee.