Buy an update for EVERY class would be murder in TF 2. Imagine having to buy the latest sniper vs spy update for a couple of items, TF2 would have one server left if anything.
Now L4D2 is different. They're adding some couple of new features, maps, weapons, probably new character and they're claiming it as an expansion. Since people are going to jump on the shinny new things bandwagon, Valve gets to ride a train full of cash.
