Same question applies for any photograph you take in public. Isn't the fact that the people are in public material to whether or not they can be recorded? That's different than calling them on their phone and recording that conversation.

I mentioned it in another reply but this is discussed in The Every by Dave Eggers, there is essentially an entire area of San Francisco that is deliberately kept free of any and all devices that can record A/V and in order to enter you have to deposit your phone and be wanded to detect surreptitious recording devices.