Why are most enamelled cast iron dutch ovens only rated up to 400-450°F? I've been investigating getting an enamelled cast iron dutch oven and am surprised to find that most of them don't recommend going over 400-450°F (200-230°C) (most even on just the base, so the lid handle doesn't appear to be the only factor there).  

Given that normal cast iron can go much higher and I wouldn't have expected that heat limitation on the enamel...what is it about the pots that causes that limitation?  What risks are there in going above it?
They sometimes have plastic (cast phenolic resin) knobs on them that are only rated for just over 200°C (400°F), or other decorative trim that is not suitable for high heat due to deformation etc.

They do tend to start crazing (surface cracking) at over 200°C too

The enamel (a special soft glass powder) is fused on at over 800°C (1470°F)

You cannot melt glass enamel on a domestic stove. It would have to get to over 1100°C (2000°F) to melt!