What are you talking about?
The disc itself isn't signed.
The firmware reports the burned DVD+R DL media as DVD-ROM. Otherwise, burned games wouldn't boot at all would they?
Editing files on the discs require either replacing them with wx360 in the image (if they are within 2048 bytes of filesize, you can probably do this without mangling the contents that follow the file) OR extracting all files from the image, and rebuilding from the ground up.
I'd have to say it more has to do with the fact that there isn't really much you can exploit with the game files, other than cheating. That and, for $2 a disc, it's pretty expensive just to test a modified file. Only someone deadset on cheating at a game, and with the time, inclination, and minor bit of knowledge to do it would bother with it. It's been done - just not widely publicized, a cheat isn't at all interesting when everybody and their pet can do it with a tutorial and a patch file.