NPS doesn't measure future goodwill (or present goodwill for that matter). Rationally it may have some correlation, but it isn't a silver bullet even when understood and used properly (which it often isn't).
Instead you often get people trying to game their local NPS scores; not quite the same problem, but similar.
> Net Promoter or Net Promoter Score (NPS) is a management tool that can be used to gauge the loyalty of a firm's customer relationships
It asks one question:
> How likely is it that you would recommend our company/product/service to a friend or colleague?
The wikipedia article provides more details including criticism.