Ok, take that back.
Apple probably do have to charge for Mountain Lion because of the Sarbanes-Oxley Act, which basically means you can’t add additional functionality to something you’ve already sold. They get around this with iOS devices because Apple account for them over 2 years in a subscription model – you get free updates because Apple treat it as if you’re still paying for it! The Mac isn’t accounted for like this so thus they can’t do free updates.
Rats. Maybe it’ll just be a token cost?