1. Look at the question the opposite way: Should a manufacturer designate the minimum diameter for a diesel-electric based on the minimum diameter of the longest cars it would be expected to pull? This would be a disservice to the customers who have shorter cars they want to pull with the PA or any other model locomotive that can negotiate O36 curves. If the manufacturer accurately labels each piece of equipment, we as the customers should be able to figure out whether we can put them together or not.
2. Sometimes what we want as customers runs smack up against the laws of physics. Expecting Lionel or any other manufacturer to design an 85-foot passenger car to go around less than an O54 curve seems to be asking them to overcome the laws of physics. As someone once said, you can't actually break the laws of physics - they won't break - you can only break yourself against them. For comparison, consider that an 18" radius curve in HO is roughly equivalent to O65 in O scale. O54 is roughly equivalent to a 15" radius in HO. The recommended radius for 85-foot passenger cars in HO is usually 22" or greater: https://www.walthers.com/85-ac...sylvania-with-decals Note that in this example, even for 22" radius the manufacturer states: "Modeler-installed extended drawbars for 22" radius curves included." And 22" radius curves in HO are close to O80 diameter in O.
