Boycotting is a form of protest of peaceful protest. Peaceful protest, is outlined as free speach. Boycotting something seldom works, but it does make people stop and think about both sides of the issue, which is really what the Pope is trying to go for here IMO. It is quite obvious that this movie and book are going to be popular, so they will not be taken off the shelves.
Here's a question for you, why can't baby Jesus be shown in a parade anymore? Isn't that free speach? I seem to find the liberals will scream about censorship to no end when it is in their benefit, but pass is off as something else, when it is not beneficial to them.