The most interesting thing about this to me is that it belies the common impression that many have that the NYT crossword is "elitist". I mean, how can it be "elitist" if it's routinely copied by USA Today??
I think people try it, find it to be impossible to penetrate on the first try, see a couple of clues about 1930's opera or NYC-specific stuff and then punt. However, like many other things in life, there is a learning curve to basic competency. There is a common, shared vocabulary among professional crossword constructors that provides a foundation for even the most difficult puzzles. The point is that you can still finish the puzzle even without knowing anything about 1930's opera by knowing something else nearby and some intelligent guesses.
Among most solvers, the quality of a puzzle is proportional to how fun it is to solve and that is closely related to non-repetition of the theme. Good puzzles minimize this and solvers take it for granted that every puzzle will be genuinely new. The best case is a theme that is never before seen, something completely original.
The problem with plagiarism in this case is easier to understand if it's seen through the lens of an inveterate solver. It's a betrayal of this unspoken (but still very casual) ethic of originality.
See almost any post from: http://rexwordpuzzle.blogspot.com/ for an example of typical daily thoughts along these lines.