Here in Japan, draft beer is called nama (生) beer, or so my boss said. Referencing my previous question, one answer said that draft beer is "pushed using gas, or drawn via a partial vacuum". I get the concept of the widget being used to facilitate that. However, canned draft beers in Japan have no widget at all! They come in different can sizes from very small to a liter size, and all have no widget whatsoever. So I wonder how it is classified as draft without having the characteristic of draft beer. Or am I missing something? Is it possible the term draft means something different around here?
Draft isn't a very regulated term but most often draws its meaning from context. At a bar, draft is usually placed opposite bottled, meaning like you said that the beer is pushed using gas from a keg or drawn via vacuum from a cask. This is the actual meaning of draft.
On bottles and cans it most often means "Like-Draft", or the marketing department's way of telling you this beer tastes more like the version of our beer that you get when you go to bar and get off the tap, but in the comfort of your own home.
For a beer like Guinness or Boddingtons, this means a widget to hold high pressure Nitrogen gas to simulate the effects of a "Beer Gas" pouring system which uses a high pressure mixture of Nitrogen and CO2. Guinness has actually replaced the widget in some of their "draught" packages as well, I believe the draught bottles.
But for a beer like Miller Genuine Draft, or the other Japanese beers you're mentioning, it just means the beer is unpasteurized which gives it a flavor more like what you get from a fresh, unpasteurized keg rather than a pasteurized and/or filtered bottled beer. This is in line with the meaning of "nama" as you mentioned, as the beer is more "fresh" than traditional bottled beer.