Just think if you would like for an atheist to start spouting off his doctrines when you were trying to shop or whatever. I'm guessing you would be pretty mad at the atheist and probably think of a couple words that would describe him, like dogmatic and intolerant.
Actually, I wouldnt be mad at all. I believe in freedom of speech. People have a choice to stop and listen or walk on into the shops. We live in a culture that preaches at us every day. The advertisements do not ask my permission, their gospel cries out from every shop window, every billboard every poster etc.
In the City people do all sorts i.e street theatre e.g buskers, musicians, circus acts etc- I appreciate the freedom of expression that is still available today to an extent.
At the end of the day if my neighbours house was on fire I would not quietly tap on his door out of fear of offending him. Neither would I ask him if he minded me telling him something important about his fate. No. I would shout "FIRE! FIRE! YOUR HOUSE IS ON FIRE! WAKE UP!! GET OUT! RUN FOR SAFETY!!!"
Open air preaching is a great opportunity to 'Go into all the world' and proclaim Christ. If people wish to stop their ears that is fine. If people choose to listen then reject it, that is also fine. At the end of the day- the sky does not ask permission to proclaim the glory of God. The stars do not stoop down and ask if fallen man would be upset if they speak of God's wonders! Day and night the earth and all creation boldly proclaims and so ought we!