the POH is not a copyright document
Now, I'm not a legal eagle so my comments should only be viewed as those of a lay person in that arena.
Having said that, I am not too sure that your statement is correct. A lot of work goes into the POH/AFM (the latter being a regulator-approved subset of the former) and I would be VERY surprised if the typical OEM did not view its manuals with a significant degree of proprietary interest.
For instance, I just pulled out a Beech POH and there, on the front page, the OEM asserts copyright.
Perhaps you are confusing the OEM's POH with FAA data ? My understanding is that the latter's publications (eg FARs etc) are public domain and anyone and just about everyone copies and sells them to the public ..
Perhaps you might cite the basis for your belief ? If I'm wrong, that's fine .. I shall go quietly and revise my knowledge bank ..
I note again non-OEM STCs generally won't be in the POH unless the owner does some footwork. Obviously the FMS should be incorporated as part of the STC mod but that only covers the then current revision - the owner has to keep it up to date - the aircraft OEM will have little, and generally nil, interest.
It is my understanding that one is permitted to copy part of a document for purposes such as research (at least in Australia that is the case). I suspect that copying the complete document might expose one to legal censure should the OEM decide to be a tad nasty.