Although it’s a bundled price, from Blizzard’s point of view, the discounted portion is the base game and the expansion portion is full price. The base game is an aged product, it’s made its money, the focus is on getting people to buy the expansion, and if the base game was still full price in that transaction, it’s a hurdle a lot of new players won’t jump. Whereas $70 is around the price for a new game anyway.
Long story short, any company would have done it the same way, regardless of what you want to chalk it up to.