Activision really want people to own Call of Duty: Infinite Warfare. While the game has seen (in my opinion, unfair) mass ridicule since it was announced, a lot of fans were excited about the Modern Warfare Remaster. The problem is that in order to get Modern Warfare, you have to buy a special version of Infinity Warfare, meaning some had planned to pickup the new game, keep Modern Warfare and trade back Infinity Warfare.
Sadly, that isn't going to work. As pointed out by CharlieIntel, there is some small print that will prevent this behaviour. A disclaimer currently reads:
Modern Warfare Remastered is a full game download (game disc must be inserted to play Modern Warfare Remastered)
This means if you buy the game physically, it looks like you won't be able to launch Modern Warfare without the Infinite Warfare disc.
I think people should have choice when it comes to how they want to play and own their games, so this is restrictive on that point. At the same time, everything I've played and seen of Infinity Warfare game is really fantastic and has me excited about a Call of Duty once more. I hope people really give it a chance.