The theory of NPS is that what matters is what people say about you. If people from 0-6 are all going to say negative things when asked, then lumping them in the same bucket is reasonable. It may not be as good as a more subtle scale, but it may be much better than thinking the numbers are linear.
It's possible that one could come up with a mapping that's even better, of course. But NPS is simple enough that even executives understand it. A marginally more-accurate number that nobody understands is probably worse, because people will trust it less. The point of the NPS score is not theoretical accuracy, it's motivating change.
