I'm not quite following the article. Why can't people build a duplex like that today? Which part of it is illegal? I think it has something to do with a zoning law, but I'm not very familiar with the concept. (I live in Tokyo.)
In most of the US most land in smaller cities are zoned for single family housing only. This often means you can only build a house for one family (no duplexes) with arbitrary restrictions on a variety of other things (minimum lot sizes, maximum house sizes relative to lot size, required setbacks from property lines to the house, etc).
In the event you are able to build a du/triplex it's rare you'd be able to convert any part of it for business use unless the land was already zoned for multi-use, where they allow mixing of residential and commercial uses.