I recently saw this notice on a glass door at UBD:
The usual wording would be 'CAUTION', not 'CAUTIOUS'. (In fact, the sign has now been corrected.)
But why? CAUTION is a noun, while CAUTIOUS is an adjective. So, why is a noun expected here, not an adjective? In fact, CAUTIOUS seems just as (un)grammatical as CAUTION.
My conclusion is that CAUTION is expected simply because that is the convention, and appeals to logic or grammaticality make no sense.