I've heard this too, if it's sent using non permeable packaging then the only way it's getting detected is if it's put through the scanners. Can you imagine the logistics of x-raying thousands of pieces of letter mail to small quantities of organic matter in the mail? consider that in the context of knowing tonnes of illicit substances are coming in through the ports. When you figure out the rationale for that let me know because it seems like a fucking waste of resources if you ask me.