Steve, I'm not real sure why but I/C rated lights (insulated ceiling) are fairly new. It is a UL listing and allowed by the NEC. Here is the web site for Halo brand by cooper lighting sold at home cheapo who say you can bury them in cellulosic(sp) insulation and next to combustible building materials.
They have a way to dissipate heat and they also have a thermal safety switch. Might want to check with the manufacture of the cellulose you are using. Maybe they are behind on their labeling or else it is one of those CYA warranties to avoid paying off. 18" is a little drastic. Even non i/c rated light cans allow insulation within 3" so go figure.