It's actually a really old practice, "the first DRM". You'd place things in your game that could only be solved by having the manual on hand, meaning you purchased it. Many games took a jovial approach to it, letting you play the game, but in a broken state if you answered incorrectly and indicated you'd pirated it. Castles II comes to mind, also Kings Quest 5. Others did the "die if you didn't have the manual", but those let you go on ... just knowing you'd lose every single time.