Why can't AGM postulates be applied to Description Logics?

Question

I hve been reading this paper "G. Flouris, D. Plexousakis and G. Antoniou. On applying the AGM theory to DLs and OWL, In Proc. of ISWC’05, 216-231, 2005." which talks about using AGM belief revision for DLs. One of the statements the make is that DLs may not be closed under negation and conjuntion. What does that mean? Won't the negation of a DL axiom still belong to the DL?