The restriction is a part of the game's 1.0.3 patch, and affects any user who buys Diablo 3 through Blizzard's online store or their Battle.net account.
The Starter Edition is a limited demo providing access up to the Skeleton King boss in Act 1 and imposes a cap at level 13. It also prevents access to the real money auction house and global play on other region servers.
Blizzard has not provided an explanation for why it is locking legitimate purchasers out of its game, but it is suspected to be a measure to counter pirated copies of the game and fraudulent access to the real money auction house. The 72-hour period will presumably to enable Blizzard to verify that each account is legitimate.
The restriction does not apply to physical retail copies of the game.
Blizzard only recently launched the real-money auction house in Diablo 3, as well as an update that will optimise the game for use on the new MacBook Pro line with Retina displays.
Diablo 3 launched last month. The game is free to users who sign up for the World of Warcraft annual pass, a yearly subscription to the popular MMO.
> Read Digital Spy's 'Diablo 3' review
Watch the Diablo 3 launch trailer below: