It's a valid complaint I'd say, for both children and advlts. Most MMOs, while fun, generally consist of the same gameplay set up in a way that it's persistent, obviously. You can drop a one-time payment for most normal games and play it as long as you'd like, most even have multiplayer that's completely free. Why should you have to pay $15 a month for what any other genre does for free (for the most part)? They get your money from the initial sale, as well as any expansion packs. Also, it doesn't cost them $15,000,000 a month to run a handful of servers and make patches (just using random numbers, $15 a month multiplied by 1 million players). Even MMOs with significantly smaller install bases are no exception to this.
I'm not against it though, I played Star Wars Galaxies and Final Fantasy XI for the better part of 5 years at $10-30 a month each, and I'll be playing The Old Republic when it comes out. Monthly fees just aren't for everyone.