[teeray]

When I explored this question a few months back, I initially thought it was to extract rent. It seems though that it comes from concerns of malfunctioning ONUs, ones that perform poorly, or those that color outside the lines of the specs causing problems on the entire PON node. It seems these concerns were originally well-founded, so the advice has been to match OLT and ONU manufacturers. Now XG-PON ONUs that you can buy are all standards-compliant, so this isn’t really a concern. However, organizations that have been burned by something in the past develop scar tissue over certain topics (“policy”), and say “never again.” I believe that’s probably what is happening here.