Apple follows accounting rules that says new features can't be added to old products without charging for them (with the exception of the iPhone and AppleTV, which Apple decided to account for differently than their other products).
They sold some Macs a few years ago with 802.11n not enabled. A few months later, Apple enabled it, but you had to buy the update for like $3 (because adding 802.11n counted as adding a new feature). People went nuts.
So now they just wait until the next paid software update comes out to do stuff like this.
Fun, huh?