It is a liability thing. It is extremely difficult to make a lightweight carbon rim with a good braking track and bead. Reason for this is that the profile of that section isn't suited to fabric lay-up and basically has to be made from solid resin. Many of the hooks on the inside are sub-standard and the tyre company will be blamed if the tyre blows off, not the rim company, hence the disclaimer. I can't blame them, I've seen some atrocious hooks and it is just spit and mercy that keeps the tyre on.