I don't think that anyone is saying that it's not possible to have a constructed fare.

But asking "how much am I paying per segment" if you don't know that you are on a constructed fare is not really a question that makes sense. And moreover, even if the fare is constructed, you have still only paid for transportation from the origin to the final destination (AS is not required to transport you through the connecting point just because you are on a constructed fare; if you want this buy separate tickets). So, you have not really paid for each segment individually, even if that's how AS determined how much to charge.

The relevant question (which, again, belongs on the AA forum) is, if you are on a constructed fare, will AA use the fare construction to determine the "cost" of a segment for LP/RDM accrual purposes, or will they still prorate by distance? I don't know the answer to this.