I agree with the 'Let them speak openly and freely.' crowd. Why? Because then I know whom to watch and to make sure I never turn my figurative back to them. It is the "smart" ones who only discuss such things in private/secret that concern me. You know, the ones who will [crocodile] smile to your face and then cut off your head when you turn your back.

Oh, and I would follow the axiom "one must choose one's battles" when trying to decide if I should say something to them or not.

And finally, with regards to private speech in a public space - I am no lawyer and this is not a legal argument, but if one has to "work at it" to hear a conversation then I would consider that eavesdropping but if the conversation can clearly be heard by the casual bystander then not so private anymore.