This is where you move into a largely grey area. Nissan arent stupid, if they were to create just one set of maps for say the UK, another for Europe, and another for the US, then thats only 3 discs they can sell. Now, should they change the software just enough for it to be legally different (forgetting the maps for now), then for their higher range cars, they can charge more. Legally, this is fully allowed, its different software, and it doesnt matter if the maps just so happen to be the same format. This would mean they have 3x as many discs as cars, much more profitable. I beleive from my economics days this is called market segmentation. Typically you dont get one customer entering the other market so they are never caught out.
I cant guarentee that all Nissan map discs will work with Nissan cars, they may have different formats from year to year. But from their point of view, what is the point in creating one format for say a Zed and another for a micra. Its just illogical, they'd have to keep converting the map data for each car.
In a sense the Nissan guys are correct also, and dont know any better. Remember they know nothing about software engineering, so to them, they recieve different discs for each car, so they assume that each will only work for that car. IF Nissan were really clever, they could even make it that way by signing each disc for a specific car and the car could check that, and inserting the wrong map disc would cause it to give an error. But this is far too much effort