There ought to be decent number of people within Microsoft who have "Copilot usage" as a KPI. I don't think this was gamesmanship on their part (no sarcasm, I truly do not suspect malice), but I'm sure if it could have slipped in without backlash, they would have enjoyed seeing their line go up.
Sorry, I don't see another plausible motive than KPIs must go up. The change came from a product manager and was also reviewed and approved by one or two apparently senior developers. They may have tried to slip it in accidentally on purpose.
IMO (and I am biased because I have written about this before in https://news.ycombinator.com/item?id=47164481) but I believe it's to make sure they're legally covering their users, and making sure users of AI tools do at least have some attribution for AI-derived contributions