Yeah that doesn't make much sense, because most MMO's are going to run you at least $50 initially, and then you pay the monthly fee in addition. I highly doubt that's what anyone would want at least with the game in its current form -- it'd need regular content updates, which I'm not even sure if they could add anything else of substantive value to a game like SimCity.
They did say it would be online-only long before launch, so anyone who paid for it knew what they were getting in to.