Books get challenged and banned for all sorts of reasons, but generally "quality" is not one of them. Often people take issue with the language used (as in Mark Twain's Adventures of Huckleberry Finn), or violence (as in The Hunger Games), or when - well - actually I don't know why on earth anyone would try to ban A Light In The Attic by Shel Silverstein. But in a country of 300 or so million individuals, no book is going to please everyone, no matter what those book clubs say. And some of those 300 million may not only dislike a book, but feel it is so wrong that other people (children in particular) should not be allowed to read it.
On the other end of the spectrum are those that get some idea that a challenged book is somehow "fighting the good fight", as if the author set out to offend people and challenge censorship (which is sometimes, but not often the case). They see, perhaps accurately, a nobility in fighting an unjust system, a la Atticus Finch in that famously challenged (and just plain famous) book To Kill a Mockingbird.
But as you consider both these sides, it's important to stay grounded in reality, and look at just which books are being challenged. As you consider books like To Kill a Mockingbird, Lord of the Flies, and A Wrinkle in Time, keep in mind that the most challenged book of 2013 was the epic, modern classic of American Stoicism: Captain Underpants.