While the chemical may be there, the tests used aren't looking for them. Just like 'shrooms weren't tested for when I was in the Marines. They have a test for them, but it costs more. With the research chems; it's because they change regularly, and most aren't technically illegal. Since it's not illegal, most people don't care to test. Just to test for "spice" you'd need to add probably about a dozen new compunds (If not more) to test for. That's a lot of extra time and expense. Both in testing and creating new tests.
A lot of folks I knew in the military (Myself included) smoked spice because it didn't show on the tests. Spice and Bath Salts were both big in their own circles due to their untestable nature. I'm not a chemist by any means. But my understanding is that a test checks for the drug you ingested. It identifies a particular compund, not the effects. Spice has no THC in it, so it won't flag on a test to detect THC.