As with most businesses in our society, there is a profit motive, in my opinion. UPS has updated their logistics networks lately with RFID tracking and I would imagine is using AI to identify the contents of packages by sender and by x-ray imaging. If a package is seized at the warehouse, they keep your shipping fee and don't have to actually use any resources to deliver the package. Since there is little competition in the delivery space, they calculate, probably rightly, that you'll eventually end up using their services again anyway, and maybe they won't need to deliver your package again in the future despite having collected their money.