That's because the Space Shuttle was designed to be a spy-agency space cargo van (with parts from nearly every congressional district to guarantee it would sail through congressional approval despite being hideously expensive and impractical.)

Make everyone go "ooo, lookie at the science teacher doing experiments to see how [plant/animal] acts in space, everybody!" so they pay no attention to the spy satellite in the cargo bay (or the different spy satellite, or other equipment, being brought back.)