Yea, K2 and bath salts are very very difficult to detect in drug screenings. The manufactures can also make one small tweak to the chemical compounds to keep it from being illegal. This is why those on probation/parole resort to these substances; generally not because they thoroughly enjoy the high, but because it's a last resort.
Referring to K2 or Spice as "synthetic marijuana" is a problem imo, too, because the effects are FAR more profound and dangerous. I'm sure you know this already, but smoking K2 alone can trigger psychosis. Just Youtubing "Spice trip gone wrong" videos is horrifying. Obviously a jump in this specific case, but homicide while under the influence of spice would be the least surprising thing ever. It literally transforms people from normal human beings to blood-thirsty zombies.
Also, I agree that marijuana can also have adverse effects on schizophrenia sufferers from what I've encountered as well.