PSN is free, if I am not mistaken, so the following only applies to the XBox.
Consider this: XBox Live is a feature for which you shell out money consistently to get Gold status and to do more online. If a true AAA game came out that allowed off-line play, Microsoft would lose a nice chunk of change in revenue from their services, because (quite frankly) there are a good number of people who would prefer to play off-line with friends for free than play on-line for money. So (and mind you this is just a far-fetched assumption which may or may not actually be far-fetched) Microsoft probably pays the AAA developer studios some fraction of that revenue to keep off-line play to a minimum. After all, both Microsoft and the developer studios are businesses, and businesses have money at the top of the priority list.
/rant