It's a very public place in the United States. It's not clear that people should expect or be entitled to much privacy in these public places.
We also know that, regardless of the degree of privacy to which people should be entitled, they're not legally entitled to much privacy in these places. Federal court rulings have been extremely clear on this point. In these places, we don't even have the right to not be photographed.
>they're not legally entitled to much privacy in these places.
While I think this is a really cool project, I also agree with the privacy issues. CA is a two party consent state, and recording a conversation (which this is likely to do) like this is likely illegal. While a person might not have a expectation of privacy about someone just hearing the conversation, they are protected by law if they are recorded without their knowledge.
NB: I am not a lawyer, and the above could very well be wrong.
Edit: As I was informed below, I was wrong on the legal points.