@mcc did you see the mealy-mouthed explanation to why it's opt-out?

github.com/mozilla/explainers/

Having this enabled for more people ensures that there are more people contributing to aggregates, which in turn improves utility. Having this on by default both demands stronger privacy protections — primarily smaller epsilon values and more noise — but it also enables those stronger protections, because there are more people participating. In effect, people are hiding in a larger crowd.