I agree that it is not unethical or illegal to copy anothers work for study purposes....as long as you have the permission the living artist and/or never intend to show it or profit financially from it.
It is OK to copy a Masterwork when the artist is long dead and there are no copyright issues.
I have seen many artists copy from the same work and the differences that I can see in the finished products have more to do with the talent, skill and the degree to which the student has mastered his/her craft.
I'm not sure how to say this but I think that when most artists (me included) paint from photos our work seldom looks like the photograph - even if we wanted it to - because we, quite frankly, don't have the mastery of our craft to do that.
In a nutshell: Those of us who work from photographs, couldn't "paint like a photograph" if we tried.
....but who's trying?