I think I haven't come to a conclusion about it, but I think it is more complicated than Strava just being opt-in.

This op-ed makes the argument that it is difficult for users and even the companies offering services to fully understand the impact of their privacy choices:

https://www.nytimes.com/2018/01/30/opinion/strava-privacy.ht...

A sort of concrete scenario here would be the app asking the user before uploading an activity and whether the user wants the activity and segments to be visible on the public parts of the service. Strava probably doesn't want to introduce that friction into their product, but maybe that is a better balance than having a setting allowing users to opt out.