The logical conclusion is that Blizzard doesn't make their revenue off of expansion pack sales, they make it off of subscription fees.
Servers and the like aren't run for free. Game developers don't work for free. Blizzard would need to get their money from something. If it's not subscription fees, it'd be them nickle and diming you to death, something complainers are ALREADY having hissy fits about in regards to completely trivial content like mounts and non combat pets.