I think I'm singularly responsible for this rule.

Years ago when I chewed, I worked for a regional that flew the Shorts. The Shorts has this cool folding "flashlight ring" in the side window, perfect for a styrofoam 'spit cup'.
Sadly, I once got distracted and forgot to remove the evidence, which sat in the sun baking for a day or two.
(Well, maybe more than once.)