Most of the tracks are not recorded by phones, but by (90% Garmin) sport watches, and the users chose to upload the tracks to Strava because of the useful features they add to that track (like comparing your exercise with others'). And the heatmap is only using the tracks that are public (again, the users chose to make them public). Ok, you don't know what other value Strava is extracting from these, but that was not a shady "we track you everywhere" business like other big names do, it's something the users really chose to publish.
