IIRC from ecma 267, there is a “control data” zone consisting of 192 repeated copies of the same ecc block (ie. 16 sectors per ecc block) repeated from sector id 0x2F200 to 0x2FDFF. (Sector id 0x30000 is LBA 0). This control zone is not directly accessible by generic scsi read commands.
In one of these 16 sector ecc blocks in the “control data” zone on dvd-video discs, somewhere in the 3rd to 16th sector contains the CSS disc keys. The 1st and 2nd sectors have “physical format information” and “disc manufacturing information”. Without going into details, some of this information is indirectly accessible by a few scsi commands specific to dvd discs.
My guess is the reason for making this “control zone” non-writeable, is such that it prevents easy bit-for-bit copies of dvd-video movie discs, where the CSS disc keys cannot be replicated exactly in the “control zone”. So garden variety dvd copiers will have to remove the title key encryption by hand, which is frequently a dead giveaway that a movie disc might be a pirated copy. Just about all the big movie companies will have a minimum of generic CSS encryption on their movie dvd discs, even if it is just “going through the motions”.