If you want a real lawyer's opinion on this, you should ask McBane or Astafas to drop by and give their legal opinions; to the best of my knowledge, they're SYM's only legal counsels.
As to how slander and defamation figure into this, they are indeed crimes but not usually invoked by street preachers.
Slander is wilfully making a false statement that injurious to someone's reputation.
Defamation is the act of slandering someone or "maliciously injuring the good name of another".
I do indeed have a problem with slander, but most corner evangelists don't engage in the practise. Furthermore, for someone to be guilty of slander, their accusor must fulfill the burden of proof and show that the defendant 1) knew that what they were saying about the other person was false, and 2) they must show that the defendant caused grevious harm or injury to their reputation. So they not only have to prove damage, but intent as well, which is a hard thing for someone offended by a street preacher to do.
For example, I believe that one of the people I work for is a complete waste of human flesh. I think that he has no principles, is morally corrupt and that if given the opportunity, he would sell his mother into slavery if it would turn a profit. It is my right to believe this and tell anyone who will listen that he is a bad person and that I don't think they should do business with him at any level, personally or professionally. However, I do not have the right to tell you that he cheats on his taxes, beats his children or poisons the neighbours pets with beef tips laced with anti-freeze, because I know that those last statements are false and that if I spread those untruths, he will suffer grevious harm to his reputation and would be the object of unfair financial and criminal investigations.
Similarly, I support the rights of people to say and believe anything they want under any lawful conditions. I support the rights of white supremecists to believe whatever the hell they want to believe. I support their right to think that black people should live in separate communities from white people. I support their right to believe that everyone else in the world is racially inferior to people of "true Aryan blood." It's the "I may not agree with what you have to say, but I will die for your right to say it" principle.
What I do not support is people saying things that are untrue (ie-a Jewish conspiracy is behind the US's policy in Israel to keep Arabs under the thumb of the west) or taking actions that lead to the imminent harm of other people. There is a fine line between the regulation of thoughts and the regulation of actions.
You use racial slurs as an example; at what point do words like "[removed]", "spic", "wop", "kike" et al become harmful? I don't know, and right now it seems that the courts don't either. On one level, they're just words ("Sticks and stones . . ."), but on another, words convey power. When a black person uses the word "[removed]" it means something completely different than when a white person uses it.
I am not sure that I am against making racial slurs illegal because I'm not sure where it would stop. For instance, if a black person is entitled to have someone arrested because he gets called a [removed], what's to stop someone with glasses from having someone else arrested because they get called "four-eyes"? Is that example trivialising racial epithets? Yes, but it's the logical extreme of where a law like that would lead.
If it sounds like I'm sticking up for the annoying fire and brimstone preachers, I am. However, I am also sticking up for you because I believe that everyone who has been the object of their ire has the exact same rights as they do. I believe in personal empowerment and that individuals can make a difference. As you say, people can change the law. If you are offended by loudmouths proselytising on street corners, then you can have the laws changed so that it becomes less convenient for them (through the due process, of course).
I am still interested to see if Guiliani's removal of evangelists on corners will hold up in court because it seems to me that outright removal of people from public places in a clear violation of the First Amendment to the Constitution, although if the city of New York's legal counsel wrote the law well, it may very well stand up to a challenge in court. Or it may be that no one has sued them yet.
In my opinion, the freedom of speech must be absolute. People must be free (by law) to say and believe anything they want, no matter how distasteful or extreme. If one person can be stopped from saying something, then we all can. And that is a road I have no desire to start down.