It's also because there are very good reflectors.....even with refractor....Dark Sky's biggest complaint was the 175 watt MV yardblasters really...that's what got them started....it doesn't matter what lamps you use really....it's the reflector assembly...
Wattage makes a difference too. If it put 100w HPS ballasts in there the sky glow would be very little too. I agree that metal halide contributes less to sky glow. Plus, the glow that is created doesn't make the sky look icky. On cloudy nights, you'll get sky glow no matter what light source or reflector type you use. The light still reflects off the road surface. i like metal halide lamp types better also because they don't make snow look like orange sherbert.
Dark Sky needs to see this pic....and EXPLAIN IT!
This just proves MH and MV use isn't a problem.