Imagine if they said paying taxes were opt-in by default. No, it's mandatory! Sure you can technically not pay taxes but you won't have a good time.
replyThat is slightly different though. The government says you must pay taxes, mandatory as you said.
replyPostHog here is saying they will train on your data but opting out is allowed. For the taxes analogy to work, PostHoh would not offer the opt-out option at all and you'd be doing something like hacking their system to filter your data out on their end.