Well, as street photo lover I simply don't agree to a certain level. I mean, I don't want to ask in advance as that would ruin the photo I would love to catch. But at the moment I know I would like to keep the picture, I run and ask. 99% of shots are deleted anyway.
BTW, I have never heard no. People mostly even give me the email (not asking for it) as they want that picture as well.