This is due to the fact that the game was developed to take full advantage of the console's cloud and always-online functionality. The policy change came too late in the development cycle to have the game capable of full offline play without a day one patch.
According to developer Turn 10 Studios head Dan Greenawalt, “So when you first boot up the game, we’re going to ask you to log in. And when you log in you’re going to get the Drivatars and you’re also going to get a whole bunch of content: tracks and cars. Our production schedule is such that we are putting them in as late as possible and that means making them free as downloadable content on Day One."
Greenawalt likened Forza Motorsport 5 to a refrigerator, explaining, "You have to fill it up with food the first time. And from then on, you connect whenever you want when you want to update your food. The Drivatars are as fresh as they are. It’s not like they’re going to degrade, but when you’re looking for new stuff – fresh stuff…it’s going to keep evolving. That’s the nature of this Drivatar system.”
One of the special features of Forza Motorsport 5 is that the game will learn how each player drives and upload it into the cloud to compete against other players - these "Drivatars" are what players race during the single player campaign.
“You do have to connect the game in order to get the latest Drivatars, because we need as many people training them as possible," explained Greenawalt, "And so rather than having just a launch-day set that was created by us, every day that people race is going to make the Drivatar set that much more accurate, that much more diverse, that much more interesting."
Microsoft is expected to announce the launch date of the Xbox One at GamesCom 2013 in August.