For the folks in BYU admin, the cost of selling caffeinated soda pop on campus would be:

1) They would be hounded by orthodox Mormons (the folks who send Holmoe letters every year complaining about the "immodest" outfits of the Cougarettes and cheerleaders) crying "BYU is condoning breaking the Word of Wisdom!" Whether or not it's official church doctrine that you shouldn't drink caffeinated pop, the myth has grown to such proportions among some segments of church membership that it has taken on a life of its own.
2) The shouts of "Hypocrites! You won't drink coffee but caffeinated soda is OK" from the anti-Mormon crowd who watch every move that BYU and the church make.

The benefits would be:
1) Not having to listen to whining from a few people about not being able to buy drinks on campus that are available pretty much everywhere else in Provo.
2) Not being accused of being liars who lack integrity by a few people who get their noses bent out of shape about this non-issue.

If I'm in BYU admin, I weight the costs vs the benefits and then just accept that I'm an unethical liar and stick with the decaffeinated pop.