When is book banning appropriate? Is it at all?

Word-counter

A religious conservatives' group in New Zealand has gotten a young adult novel banned because of this:

Your correspondent having been thwarted in bringing those parts of the "Into the River" that might have worried members of Family First to the attention of readers of Prospero, found that Mr McCoskrie had industriously done some counting. He said "it's a book that's got the c-word nine times, the f-word 17 times and s-h-i-t 16 times." 

The Economist's reviewer zoomed out to a less granular appraisal:

The book is about a Maori boy sent to an elite school. It has passages of sex, swearing, bullying and drug-taking. The author, a teacher, says he based it on activities he had observed in schools. He was profoundly upset by the ban, and the hate mail he received afterwards. The book had won two major awards.