The box office success of 'Sound of Freedom' — an action film about child trafficking — really shouldn't be a big surprise

• "Sound of Freedom" is a box-office hit, taking in $90 million since opening July 4.
• The movie is a huge success for an indie — and there's clearly a market for this kind of content.
• Movies marketed at conservative Christians have also done well in the past.

"God's children are not for sale," says the hero of the movie "Sound of Freedom," the box office hit about a former Homeland Security agent who goes rogue to save children from sex trafficking.