That arrangement is partly custom, but also is required by law. All foreign companies selling products in the US are required to have a US agent, mostly someone or some company the law can grab if necessary. In recent months, Toyota USA took a lot of heat, and BP USA is now really feeling it.
In firearms, the US agent/importer also is the point at which BATFE begins a forward trace when required; trying to do so from a foreign company that might not even be in business any more would be impossible.
In commercial trade, the agent/importer's name will be on the gun, along with that of the maker. On milsurp guns, which may not have had the name of the maker or only a code name, the importer has to stamp his name for tracing purposes.
Obviously, such rules didn't apply to the millions of guns brought back by American troops from several wars, or to guns imported (legally or illegally) by individuals.
Jim