Isn't it "trivial" to represent/reduce any classical physics problem into a Spin-Glass which is NP-Complete? In the late 80's there were several highly cited efforts to use Spin-Glass models to formulate other computational problems such as: Protein Folding and Neural Networks. 

Isn't it straight forward to reduce any classical physics problem into Spin-Glass which is NP-complete problem? 

What I am saying is: since Spin-Glass is at least as powerful model (computationally) as any classical physics model it is always possible to restate a classical system using Spin-Glass.

I believe I am missing a huge point here, because these 2 papers are highly cited ones.

EDIT: If you downvote, please, kindly consider to add a constructive comment.
Classical physical problems often involve real-number positions or parameter values rather than values from a discrete set (such as the integers) which would be more typical of NP-complete problems. Because of this these problems can often be NP-hard but not (or not obviously) NP-complete. For instance, we do not know whether the Euclidean TSP (for integer points in the plane, with Euclidean distances) is NP-complete, because it hinges on an unsolved problem, the complexity of comparing sums of square roots. Some other geometric problems, such as the recognition of unit distance graphs (of some relevance as an abstraction of protein folding) are complete for the existential theory of the reals, which as far as we know is a different complexity class than NP.