But when a disk has an OS predating the one on the hard drive it still may be possible. The most salient example is Disk Warrior. These guys are never in a hurry to update the OS on their DVDs, yet their SL DVD that I just got from them boots up my MBP with Lion.
Apples and oranges, of sorts. Obviously, if a(n Intel) Mac that shipped with let's say Tiger was upgraded to Lion, it would boot from all generic OS versions following the one it shipped with because they all contain the required hardware drivers. It doesn't work the other way around: no Mac shipping with Lion can boot from a Tiger OS disc, because the necessary hardware drivers did not yet exist when Tiger was released.
As to Alsoft's policy regarding updating the OS on bootable DiskWarrior discs, that's essentially governed by restrictions imposed by Apple regarding OS updates included with DiskWarrior. DW updaters typically use the OS version installed on the (required) original DW Installer disc to generate the updater disc. In contrast, DW upgrade discs are shipped with a current version of the OS. Depending on the availability of the system startup files required by new Mac models, you may not be able to boot such Macs with the then-current DW disc.
