tis a bit odd isnt it.
being cynical, to me it implies one of 2 things:
they dont think WoW is good enough to sell on its own merits.
they dont think Diablo 3 is good enough to sell on its own merits.
now, i think the real point is, they know that the way things were going they were going to lose 50% of their subscribers for a month or 3 while they all went and played D3 - and by doing this it means that people can more easily afford to play both wow and d3, and thus their subscriber numbers wont take a massive hit (the problem with a massive 'temporary' hit like that being that not all people will return, especially once you consider the number of raid guilds that get screwed by it).
so yeah, i dunno, can see it either way i guess.