Just because you have an XBox doesn't mean all games will have free online content.

Games that are designed to be online only or primarily online will have monthly fees. It's no different then PC games like Ultima Online, Everquest, Dark Age of Camelot, etc. EQ on PS2 will have a monthly fee, and Star Wars Galaxies on PC and PS2 and Xbox will also have a monthly fee.

It costs money to keep servers running to host thousands of players.