Opportunity cost, possibly. They might be remastering it and wish to sell it at a higher cost, or think they can get more money through a different retailer.

In this case you can’t play your purchased dlc (or online multiplayer) but you can still play your game. Games affected: Anno 2070, Assassin’s Creed 2, Assassin’s Creed 3, Assassin’s Creed Brotherhood, Assassin’s Creed Liberation HD, Driver San Francisco, Far Cry 3, Prince of Persia: The Forgotten Sands, Silent Hunter 5, Space Junkies, and Splinter Cell: Blacklist

There are games and software that check a server to see if you are entitled to use it when you run it. If that server goes down or they geo block you, or ban you then you may not use the game or software you purchased (unless you crack/pirate it).