I've also stumbled across the odd duty free that has this restriction on alcohol but is happy to sell the other products (e.g., mainly beauty products) to domestic travelers. I think this is the case at smaller airports in particular like my home airport, RDU, which makes sense given there are only 2 daily international flights (with some additional seasonal international flights sprinkled in to KEF, CUN, and NAS) so they aren't seeing significant volume on a daily basis