Guns are different than speedometers.
A .308 is made to shoot .308's, a 12 ga is made to shoot 12ga shells, a .45 colt is made to shoot .45 colts, and so on. Any J Frame, L Frame, K Frame, N Frame, etc. made in the .357 Mag caliber should be built to shoot that caliber throughout it's lifetime, assuming "normal or standard" pressure loads are used.
If S&W say's the gun is not able to do so, it's borderlining on legal issues, they should extremely careful in their replies to customers. Again, if that is the case, they should be taking steps to solve the problem.
The average Joe that decides to buy a .357 J Frame for his first gun, buy's it with the obvious assumtion it's made to shoot .357 mag ammo. He may not even know you can shoot .38's in it.
My 02 cents.