You guys are talking about increasing sales by a small number if they buy the photopass CD. Not that many people actually buy the CD. That is part of the significant cost of it. No matter how many buy, the cost to the equipment and the photographer staff throughout the parks is fixed. Since a small percentage actually buy the CD, each has a high cost. Lots of people take the photos, but once they get home and look at them and then the $150 price tag, few buy it.
However, the on-ride photos are nearly free to disney after the initial hardware investment is paid off. Lets say that the Photopass CD has a cost to disney of $50. (just a guess, but since you can pre-order it for $99 and Disney is certainly not losing money on it, lets guess at $50 cost). Lets also assume that the on-ride photos will not significantly effect pre-order sales. So, every extra CD they sell is $100 of additional profit.
Not knowing exactly (or even how to guess) what the cost to Disney is on the on-ride photos, but it will be relatively low; lets say $3. They need approx 8 impulse sales of the on-ride photo for each extra Photopasss CD they sell strictly due to the on-ride photos. Now spread that around to the other rides as well... ie the same person may buy a on-ride at splash and ToT but in a best case could only buy 1 photopass CD. What you get is a significantly better income from selling the on-rides separate from the photopass CD.
Now certainly my numbers are off. They are only guesses for the sake of comparison... but even at 10-1 or 15 -1 Disney's $15 extra charge is a big money maker.
Its called incremental expenses. It is why many of us use credit cards. It is why payments are spread out over a period. It makes you think you are spending less, and the mental decision to open your wallet 10 times at $15 each is far simpler for Disney to influence than to convince you to open your wallet once for $150. Then again, some people may even do both the CD AND 1 or 2 on-ride photos.
If you don't look at all of Disney's decisions in this way, then you will always be frustrated and disappointed in the decisions that Disney makes. I promise you THIS is how they look at it. As for me, I have never bought an on-ride photo, and don't imagine that I will ever. The marketing is to use the adrenaline rush, or the euphoria that lingers from the ride experience to lower your guard and make you purchase with less thought. Its brilliant marketing.