I think the case that folks are trying to make is that you, as a TSO, are making a preliminary determination that a substance merits further examination.

Just to be ridiculous about it ... if you open a checked bag and see a polo shirt, you don't typically call a LEO over because you suspect that the shirt is, in fact, a cleverly designed stash of drugs. You look at the shirt, you say "gee, that's a shirt", and you move on to find the item that caused the alarm in the first place.

On the other hand, if you open a checked bag and you see a large sealed bag of white powder, you make a judgment that the item might be worth having a LEO look at, and you make the referral to a LEO.

You're not making the final judgment as to whether or not the item might be something whose possession violates state or federal law. But you are making a judgment as to what items to refer to a STSO or LEO. And, one is naturally curious as to how you make that judgment, since that's the first link in a long chain of events that can lead to difficulties for a passenger.