Companies who mail free samples often will not accept addresses that are PO Boxes because they are either trying to restrict freebies to one per household or they are sending freebies using a private mail service (like UPS or FedEx).
When you have a PO Box it does not tie you to a household, which means a household with ten PO Boxes could request ten freebies sent to their ten different PO Boxes. All freebies are limited in quantities. When someone takes advantage of the system in this way the company misses the opportunity to send out 9 samples to 9 other households. By restricting the sending of freebies to only physical residential addresses, this helps reduce the number of people who request multiple freebies.
When companies send freebies using a private mail service, then they are unable to deliver anything to PO Boxes because they don’t know where they are located. Only the United States Postal Service knows where the PO Boxes are located and can deliver mail and packages to them.
I hope this answers your questions about getting freebies at a PO Box!