[Kylotan]

That's not entirely true. It's actually more in their interest to keep you mildly interested, with some community ties or other investment in the game, enough to play each month or at least renew the subscription, but not necessarily to play for 8 hours every day. Since they charge by the month rather than the hour, they make most money when a subscriber is not playing, as they then incur no overheads on bandwidth, cpu, customer service, etc.

Personally I don't think there is anything particular to WoW that is any more addictive than most other online roleplaying games. Blizzard want players to stay interested but I doubt they ever plan it in terms of 'addiction'. That's more the external view on it, and is . With something like WoW you get pretty much the same gameplay as MUDs have offered for years, except you pay a premium for a much larger community and a whole load of polish.

It's a business. More players means more revenue and more financial safety for the company. It keeps employees employed and it keeps existing customers happy. Why on earth would they not advertise?