Many people get 100-200+ hours of entertainment from WoW every month. Any other form of entertainment would be much more for that duration of time. Do the math- $15/200+hrs=$>0.075 per hour, or just $0.15+/hour for those who don't play as much, but still play quite a lot. Many hardcore WoW players play even more than that, but because there are many "casual" WoW gamers, it lowers the average. Movie tickets cost ~$10, plus the cost of food, and they're only 1-3 hours long, usually around 1.5-2.0. That's ~$5-7/hour, not including food and drinks. Hookers are even more expenisve. Waaaay more expensive. Also, STDs.
The bottom line is that WoW is fun and cheap for the amount of enjoyment people get out of it. You don't have to play it if you don't want to. Let WoW players play WoW and STFU if you don't like it. End of discussion.
One last thing- Blizzard is a company that develops online games. Wow is very bandwidth-intense, and it isn't exactly free to have a player on 200+ hours a month. They need the $15 a month to not only pay their employees and bandwidth fees, but to also help fund new projects to keep their players content. Example: The Burning Crusade.