It doesn't have to be set down in a contract, but if you explicitly chose to give someone permission to do something, it would be petty to turn around and complain when they did exactly what you told them they could do. This isn't some little loophole Sony's exploiting: It's the largest distinguishing feature between BSD and GPL. If the NetBSD people have a problem with this, there is nothing stopping them from switching to GPLv3 or MPL or anything else. But it looks like they don't actually have a problem with it, and the HN title is editorializing.

EDIT: For posterity, when this thread started, the title was something like "Sony uses NetBSD code in PSP, refuses to contribute back".