I agree with poster #1 as far as the American Library Association is concerned. Perhaps you are confused because they release a list of banned books every year? Also, it does not appear that Main Street met with much opposition. The town of Alexandria, Minnesota banned it in 1921, although it was actually based on a nearby town called Sauk Centre, Sinclair Lewis' childhood home.
In the novel, it portrays an emptiness & restrictiveness in small-town life. Although at the time, many people considered living in a small town to be the ideal, Lewis challenges that with his study of the effect on its residents, especially women. As the main character defiantly states at the end: "I do not admit that Main Street is as beautiful as it should be! I do not admit that dish-washing is enough to satisfy all women!"
I do not believe in banning books of any kind. I think keeping age-appropriate texts in a school library (particularly elementary) is a different matter entirely. I don't consider that "banning", although it becomes tricky in deciding what's appropriate and what's not. It's when people are offended by certain material, & they can't reconcile that with the educational nature of a text that problems occur. However, I feel that especially in high school, students should be free to make their own decisions about texts, rather than having them made for them. Also, many people attack books without even reading them; they simply hear from someone else that there's something offensive in the book. Also, I think as a country we're becoming too insular. We're afraid to read any opinions that don't match our own. Instead of opening a dialogue about difficult subjects, we engage only in conversations that will validate our own views. It is important to read literature that shocks, offends, and stimulates us, because that is how we formulate our own thoughts on life.
I do not believe that you are correct about who the book has been banned by. The American Library Association fights against book banning and keeps lists of books that have been banned. It encourages people to read them.
Some small towns have banned the book because it portrays small town life in a bad light. It makes small town people look like small-minded hypocrites at times.
I can see banning books from children's libraries if they have sexual themes for which the kids who go to the library are too young. But it seems silly to me to ban a book because you don't like the way it portrays people in towns like the one you live in.