Revealing mistake: In the scene where David starts the game with Joshua, it asks him what side he wants to be (1 for the U.S. and 2 for USSR). Look carefully at the closeup of the computer screen - his choice, 2, appears long before David actually presses the key.
Revealing mistake: Where the officer is pushing the buttons on the wall panel to launch the missiles, on the both occasions you see it, the panel buckles highlighting the flimsy set.
Revealing mistake: On the large screen at the end of the film, Joshua and Prof. Falken are 'writing' to each other. Falken types in "Hello Joshua"- he not only says it, but you can hear him typing it as well, however, on the screen, only the word "Hello" appears; the word "Joshua" doesn't.
Revealing mistake: When Joshua/WOPR is trying to figure out the launch code at the end of the movie it picks the "1" out of the 1704 twice.
Revealing mistake: When David begins to play against the WOPR computer, the images on the war room screens show various tactical movements by other countries. These are the same images shown the first time the war room is seen.
Answer: While merely speculation, the WOPR is not alive and knows only what it's been programmed to do. It would have no concept of life or death, and as such would see no difference between the simulation and the real thing. That being said, an easy way to make it see the difference would be to program it to not waste physical resources. It would then see the use of all its actual warheads as less desirable.