But that "everyone" isn't a single entity recording everywhere in public with the intent of providing tracking information of everyone else. If I'm in the background of someone's selfie and posts it online, it could be used to get my location at a specific time, sure, but their intent wasn't to do so and the scope of their recording is dramatically more limited than Flock.
To add to this: you being in the background of anyone's selfie is not a problem of itself. It is the aggregation of everyone's selfies that is a surveillance hazard, the risk associated with each individual picture is minimal.
But that "everyone" isn't a single entity recording everywhere in public with the intent of providing tracking information of everyone else. If I'm in the background of someone's selfie and posts it online, it could be used to get my location at a specific time, sure, but their intent wasn't to do so and the scope of their recording is dramatically more limited than Flock.