I would probably react the way you did.
But I wonder if such a reaction isnt just to the novelty of the situation.
That is, if this kind of behavior occured regularly, would we just not notice it anymore and go on about our business?
The insight of that question for me is that it points out there is no intrinsic problem with what they were doing. Such behavior impinges on us only because of its novelty, not because it is actually harming us in any way.
Therefore, my thoughts are moved on to why is such behavior proscribed. Are they proscribed because of any intrinsic factor or is it just that "someone" in the past got a burr up their butt about it? I would draw an analogy to the recent spate of threads on how members here should act this way or the other. Where, from the perspective of the OP, some behavior ought to be condemned because the OP didn't approve of it. The discussion eventually examined the logical conclusion that everything would be banned if we had to listen to everybody.
This answer may have seemed to have wandered from your original post, so let me bring it back.
Is it their behavior that is a problem or is it our reaction that we have been taught that is a problem? If we just got over our adversion to PDA, would the "problem" go away?
Perhaps it is as Pogo said: