That is ridiculous...... Insurance should be used weather discussed or not, it's common sense. If the postal service damaged an item during shipping and you did not insure it then you're out of luck, and that's on you. Same concept.
Ridiculous or not, there is no real way to force people to do it. That is why it's strongly recommended, but ultimately it's left to the adults doing the deal. We cannot be intervening on every transaction that happens. The overall expectation is that the parties involved in any transaction must be clear on what they will do, that is common sense. We also expect that if a person is going to trade an item, yes they should be 100% sure they want to trade the item. In a sale situation, they should verify that they can financially afford the item before committing to buy, or be sure they want to sell the item to begin with; again more common sense.
The rules are in place as a guide to help folks conduct things in a fair and honest way. A lot of thought was put into them and were written in such a way as to not be encroaching, but helpful to those active in buying, selling, and trading. Knifehunt, check with the post office, give them the tracking number and let them know the item was not delivered to your mailbox. At this point, that is the best course of action I can offer you.