I doubt your cuffs lacked an interlining. If they did, they would fall apart and fray pretty quickly. I would call that a mistake on the maker's part, not a design decision.
The stiffness of collar and cuffs comes much more from the inherent stiffness of the interlining itself, and less from whether it is fused or not. Fusing adds some stiffness, but a hard unfused interlining will be stiffer than a soft fused one. Shirtmakers that I have used tend to use the same interlining, or perhaps offer one or two choices so that the customer can specify "stiff" or "soft" as a general matter. If the same interlining is used in collar and cuff, and the former is fused and the latter is not, then the cuffs will feel bit softer.
Personally, I hate fused cuffs, but don't mind fused collars from certain makers. I would not say, however, that an unfused collar is "wrong." But it is much easier to press an unfused cuff than an unfused collar, which is why a lot of people prefer fusing in the collars.