I think that street preachers violate the “do not cast your pearls before swine” dictum. They have a right to do what they do, but I think they do more harm than good.
It is possible that they do this (in my opinion) if they are rude and call names and do not have the love of Christ. Even John the Baptist and the prophets of old who warned the people of sin, impending judgment, and guilt before G-d showed love.