It's not that it can't be burned onto the DVD-R. It is in fact burned onto the DVD-R but the signature is only valid for the media MS signed it for. Basically that signature will only let the xbox boot on pressed DVD-9 Media. I'm pretty sure that pressed and burned discs are identifiable and would been different signatures as well. I don't know that one for a fact but I'm sure someone here has burned an xbox game on a dual-layer disc. (I'd be willing to bet that it didn't work)
If you are skeptical, there was a mixer program or something that was signed on cd-r. It could be burned onto a cd and play in any xbox. (Unless you had a thomson

)