It's illegal (and against USPS Policy) to ship beer/alcohol from non-licensed retailers. I don't believe BiaB, Etre, or any of the Belgian sites are licensed retailers in the US. Plus when you buy from these sites you're technically not paying the proper customs. Then there's the whole shipping across different states and how these boxes are skipping all the interstate laws/taxes that apply to alcohol. Basically blame prohibition.
There are really only two legal ways to ship alcohol in the United States. A retailer must be licensed to sell alcohol by a state that permits shipments in and out of that state and must have an alcohol shippers contract with either UPS or FedEx. The US Postal Service does not permit shipments of alcohol. It is illegal for a non-licensed individual to ship alcohol. Additionally there's like 15 states (TX, PA, MA, etc) that its illegal to ship any type of alcohol to.