That would be cool, but it might cause a bit of congestion. All the people coming off rides would be lingering around at the ride photo counters to swipe the photo pass cards.
Also I imagine this would not be a profitable move. Now I'm just speculating, but I'm guessing that most people who had their photos taken in the parks prior to photopass never took time out of their day to go back and look at the pictures later on. But now that they are viewable online probably most people view their photos once they get home, therefore increasing sales.
But with the ride photos, you can see them right there on the screen as you leave the attraction. It only takes a moment to decide if you want to purchase or not. If you don't purchase, you won't be able to see the picture again, so that's a good incentive to make the purchase. But if Disney posted ride photos online then it would just cause people to not purchase the photos since they'd be able to look at them anytime. Does that make sense?
Again, this is all speculation.