I've only ever seen full character cases go to European/UK stores. I don't know why they are different over there, but maybe some European retailers will only order certain characters, rather than whole waves.
I would assume globally Amazon gets single-character cases to make their in-warehouse logistics easier to manage. With their massive turnover in their warehouses, there would be little time to teach a picker the difference between each of the figures in a regular case.
I have a list of all G1 characters that have been released in CHUG form. You can find it here. Please feel free to let me know if I got anything wrong so I can fix it.