They do care.
They want to make a profit, and every account counts towards that end.

If you cancel your accounts, they want to know why.
Presumably, so they can change so others like you won't cancel too.

That said, any given account is a drop in the bucket.
If you do cancel, it isn't really going to be hurting Blizzard by any large amount.

Ultimately, they'll do whatever is in the best interests of the game, in terms of what will get the most subscription dollars.