Really? What if they're just out walking and are unaware of the existence of other humans? Finding a city when you think your group is all there is would be rather "contaminating". And you've still contaminated that one; if they bring back that information, you've allowed contamination of the culture as a whole, the same as if you went yourselves, except you get to sit back and lie about how you had nothing to do with it. Unless you wish to state that an individual can and does accurately speak for a group, especially a group you know almost nothing about except what that individual has told you? What if they're the anarchist of the group?

And if we were to know if they think other humans than themselves / those they immediately interact with exist, we've probably exposed ourselves to them, as that would be hard to know through un-detected observation alone.

Our existence is interference if they become aware of it. Absolute non-interference is impossible if they ever become aware of us in any way. If we "must not contact them" unless they want it, we cannot impose ourselves on any who do not wish to contact us, which means complete isolation until they magically express their desire to contact that which they cannot know exists.

My point is that "We must not contact them, unless they want to contact us." is a logical impossibility, not something one can actually achieve, and it makes no sense to hold such an impossibility as an ideal.

"What if they're the anarchist of the group?"

What if they are? Can you see the diference between giving them, a group with far fewer resources to defend themselves, the chance to remain isolated if they want it so, or contact us as well, against forcing our presence there?

"Our existence is interference if they become aware of it"

I concede you that point. Can I rephrase "must not contact them" with "should minimize points of contact from our side"? Those planes have their purpose: to make us aware of their presence, because we're about to wipe out their environment. Should we keep our distance? Yes. Should we do nothing and let companies run them over? No, absolutely not. Then I go for flying over them and take pictures.. It's the lesser of two evils.

There's no magic. Once they discover us, instead of we discovering them, they can take the step to communicate wth us.