I don't know if there is a code requirement for the "grounded plastic box". Those boxes you are talking about are the ultra cheap blue boxes that are found in the big orange store. My theory on them is that they are for light fixtures that don't have a ground tail on them thereby grounding the light through the fixture screw. I haven't ever used them. I am not sure what purpose they serve because almost every light I have ever seen has a ground tail already attached.
I had the same culture shock when I went to New York to visit. I felt like I was in another country with all the se cable and the meter bases on the sides of houses, and the meter/ disconects on the outside of commercial buildings instead of switchgear, it was real neat to see all the differences in the way things are done.
