Because the time that needs to be invested into a War3 level is minimal compared to the time needed to design a level in 3D for Q4. Blizzard have released MORE than id or Raven in Q3 or Q4, but I'm not going to neccecarily commend them on that because you can't compare the two.
Yes, you're right, the time should be a factor. But as a factor, it can be scaled into the decision making process relatively easily.
The reality is, however, that when it comes down to it, I wouldn't have much of a problem with what they are doing if they had have started with a somewhat substantial content addition. Asking people to pay for horse armour struck me as almost insulting. It definitely set a negative tone to the whole thing, which they seem to have realized based on the subtext of their announcement.
I would, of course, much rather they simply put the content into an expansion pack. Hopefully they'll do something like that (package up all the extra content in some fashion).