@Tempes

You can't buy the mount(s) unless you have a battle.net account, I believe?

All expansion packs clearly, I believe, state that the original game is required - which, when purchased, came with some free game time (though I'm not sure there's any 'legal' requirement for said game time to be included, it would seem daft not to do so) - so there's your free online service subscription time.

So each expansion pack is, effectively, an upgrade. Like when you buy Office 2010, for example, which can come in two flavours:
1) Full retail price for the whole thing
2) Cheaper upgrade price, for when you have a previous version of Office, say 2003 or 2007.

To turn that into wow/blizz speak, substitute Cata for Office 2010, Wrath for 2007 and BC for 2003.

There are many instances of things being sold - and/or licensed for use - that require something else to already be installed or purchased. And they /can/ be purchased separately, but they won't work. Things like covers for phones, chargers for electrical items - hell, even electrical items themselves (eg I could walk into any electricity supply company's shop and buy one of their products, but if I didn't have any electrical supply, I couldn't use it...)

But that's not the case with mounts - at least as far as I can tell. You can't buy one without a battle.net account, so you couldn't buy one unless you could use it, and you can only use it when you have plugged it in (entered the correct code) and switched it on (connected to the servers).

As for cell/mobile phones - I can buy many, many SIM-free phones, from very cheap to arm and a leg expensive. But it's just a paperweight until I sign up to a phone service provider.