They don't want you to "contaminate" the "specimen" for one thing, and then if you try to flush or wash, they could say that you tried to "alter" your specimen.

I washed my hands BEFORE they let me go into the toilet, and was told not to flush or turn on the water. The Pee Police are afraid you'll alter your speciment by adding water, or something. i forget why they don't want you to flush...Old Timers Disease...He, he, he....or could it be short term memory loss from all the THC, LOL?