shorten NBS AC cable

I suppose it has to do with the design/build of particular cables. With something like the Shunyata, the insulation includes material that absorbs stray radiation of RFI (the way ferrite beads do the same), so shortening the cable will reduce the benefit of that kind of treatment).

Also, because the manufacturer cannot know what kind of skill is possessed by the person reterminating the cable and the quality of parts that will be used, I can see why they would recommend against cutting the cable even if there were no design considerations.