One reason is the extra instructions are an artefact of the decode process - undefined behaviour that just happens to be useful - but woe to you if you rely on them (although I have heard sometimes they are so useful they get publicly supported, I think something like that happened with the z80 and something to do with addressing/address space).

But here, it's weird, especially something like division which presumably isn't cheap to implement and is useful to end users. I don't get it at all.