I haven't read the book.
I can't imagine an appropriate context, can you?
I couldn't either, until I sought help from Google. It appears that this is a Vietnamese cultural thing. So, expect the 'Rats to defend Webb by saying he was just practicing the novelist's craft by adding vivid details of the local culture — which he picked up while serving his country. The book got good reviews, by the way.
It's good that the story is getting out — maybe some 'Rats will change their votes or stay home. But it was probably a mistake for Allen's campaign to touch it.
It seems Vietnam is not the only locale in which this practice is regarded as normal. In Maine a child abuse conviction was overturned on the basis that the behavior was OK in the defendant's culture (Afghan in this case).