PC vs. Fair Notice and Retroactive Legislation

I believe that at least some of the hostility to PC, especially from those otherwise inclined to support civil rights causes on the left, comes from the same impulses that underlie the (legal) doctrines/maxims of 'fair notice' and against/prohibiting retroactive legislation, and not without reason.  Many standards, like against micro-aggressions, erasure, and invalidation, and other forms of discursive violence, are not only higher than they used to be--which is certainly very plausibly the right thing to do--but vaguer and more fluid than they used to be (both there's more variability of them between different people, and they change more/faster over time [I suppose, for each person and for populations as a whole]).

Conor Friersdorf talked about "concept drift" (I think) in The Atlantic.  

Catherine Rampell reported on "sombrerogate" and Bowdoin.  Her second report included the detail that the school administration itself had committed the same discursively violent violation itself only the previous year, with even additionally offensive fake mustaches.  

Conor also reported on the case of the micro-aggressing email and response.