...People are misunderstanding.
Faith and espionage do come with Brave New World, yes. Why? Because they are core game mechanics that need to be intertwined with BNW content. Making it entirely separate from the core concepts G+K brought into the game would be ludicrously inconvenient.
Just because BNW comes with G+K mechanics does not mean, in any sense, that you're somehow "paying for the same thing twice". You're paying for all that BNW introduces, the truckload of new mechanics and civs and wonders and scenarios. That stuff is well worth the money you're paying, if it's the same price as G+K was. The fact that the expansion includes the mechanics introduced in G+K is just icing on the cake, allowing for this expansion to incorporate them better, and allowing people who only recently purchase Civ V to need to buy one expansion to be able to play with the larger, more dedicated community.
At this point, what's likely to happen is a drop in G+K's price, since to most people it will be simply a civ and scenario pack. Granted, those things are well worth money too, considering that a LOT of production hours go into leaders.
From a marketing standpoint, it is simple. People will want to buy Brave New World more than they will want to buy G+K. However, G+K is still a separate product that represents a good portion of the company's hours put into work. They can't simply include it into Brave New World, since A. it would be a good marketing plan for people who only now bought Civ V who don't really want to spend a load on new expansions to offer them just G+K at a lower price, and make a bit more money while those people can play the older expansion alone and judge whether they want to get the new one, and B. doing that would be like selling a sequel to your book that includes the first book. It makes no sense for you to do. The sequel certainly incorporates elements and improvements from the previous book, such as the increase in refinement your writing has achieved from initial publication and the broader scope of the story's world, but you still want the original book to continue selling. If people complained that buying the sequel to the book means they should get the first one free since it means they're paying for the elements the second book brought over twice, they would quickly be dismissed for being silly. The best course of action for everyone involved is to continue selling the first book at a lowered price, while focusing marketing and income on the buzz over the sequel.
