"Buy one get one free" is just 50% off.
It's all taken into account before hand based on reasonable expectations to insure that the proper amount of money is taken in while still allowing you to feel like you saved some cash.
It just depends on how you look at it.
[snipped analogy] Your analogy made no sense.
The buy4 get 1 free is an incentive for you to purchase that 4th picture.
I don't know the behind the scenes profit margins but I would assume Disney makes slightly more with that package than if you just bought 3.
Did you think Disney was offering it to be nice?
As soon as you purchased your picture package you bought 5 pictures.
What if Disney went on to say that the conditions under which you bought them requires that only *YOU* can be in the picture, not any other member in your family.?