I agree with everything you said. +1LEO asks; "Do you know why we're here?"
In several recent threads, both here in Washington and in other states, I have noticed encounters with LEO asking this question. In all of the threads the OCer has responded.
I think a fundamental re-thinking of this question needs to be taken by those who have answered 'yes' to the officer. Why would you answer yes?
If I was asked this question I would respond, "No, officer, why are you here?"
The reason being, is that I am doing nothing illegal and thus there is absolutely no reason to assume the police are 'here' for any reason. When an OCer answers to the affirmative they are playing into the officer's notion that something bad or illegal is going on and thus the presence of the police is needed to deal with the issue. Regardless of how the LEO came about the OCer (MWAG call or circumstance) the only answer that is appropriate is "No, officer, why are you here?" or some variation thereof. It thereby reiterates that nothing illegal is taking place and puts the officer on notice that he must articulate a reason his presence is needed.
Of course after you establish this as the basis for your interaction the LEO will move his/her technique to obtaining your identity, in which you should immediately answer with a question.
"Am I being detained?"
"Am I free to go?"
"Am I being arrested?"
"Why am I being arrested?"
And unless the answer to the question places the OCer under arrest or detainment you must be free to go and the OCer should immediately terminate the voluntary encounter.