id Software sent out letters/e-mail telling sites to take down their copies of Quake 1 for Dreamcast. Even though the Quake source was free, the game content was not. The copy of Quake being spread around contained game data from the full/commercial game.
Since it is a direct PC port, the data can be messed with in the same way. Quake 1 for DC has been "re-released" with the Quake 1 Demo files (the first "episode"). There is also instructions at DC Emu on copying your full Quake 1 game data to the Dreamcast CD to return the game to the full version.