The Blizzard that made Diablo and Diablo 2 is not the same as the Blizzard of the today, the key people have moved, so I don’t see the issue with playing their older offline games.
Ironic, the mention of those two. They've both been (or are about to be) re-released (and - as I've heard - have replaced the originals in the launchers). As such, they are both still revenue generating products for ACTI.