Well, maybe because they have a great sense of irony? I've found, over my years of teaching, that it doesn't take much to challenge a book. On more than one occasion we've been told we cannot teach a particular novel, even when the School Board has not read the book themselves, and is merely taking the recommendations of a small committee with a private agenda. Book banning, I think, is most often the result of the lack of critical thinking Bradbury warns about in this story.
The American Library Association has a great list of the 100 most common banned books and statistics about the reasons books are often challenged:
The most recent documented challenge to this book was in Texas in 2006. Parents of a high school girl complained not only because the book contained curse words, but also because it discussed drunkenness, "dirty talk", and referenced the Bible disrespectfully. Here is a link to the news report about that:
First, let's talk about what it means to "Ban" a book. It doesn't mean the book was actually illegal to read. Books are usually "banned" from certain school districts, so the kids can't read them in school. Sometimes a few libraries will refuse to carry a certain book if they believe it is too offensive. Fahrenheit 451 was banned from a school district because it used the phrase "God damn!" The school board felt that this language was inappropriate for students to read.
The reason that we hear so much about Fahrenheit 451 when talking about banned books is because that is what the book itself is about. It takes place in a futuristic setting in which the government bans anything that might offend anyone. Books are illegal because someone might not like something that is in them. Television presents mindless dribble intended to keep everyone happy and entertained. People are kept from thinking for themselves or worrying about anything.
To summarize, a school district found some language offensive and so banned a book that was about the banning of books because people find them offensive. It's nearly poetic, don't you think?