You're taking it too far there. You're trying to apply reason on top of what you describe as arbitrary.

I can't conceive of all the reasons why DL, by rule or by search engine quirk - doesn't show awards between two points, but I observe it routinely enough. (Try finding an award on China Airlines LAX-TPE-DPS and go mad.) Delta, unlike, say, a Federal Appeals Court, doesn't take 150 pages to explain in cited detail why you lost. The business is selling tickets and transporting people, not explaining why we can't use miles in specific circumstances.

Do you have a way to get FlyingBlue miles? Try an award search on the AF/KL Flying Blue engine.

For situations where the alternative to an award is a particularly expensive cash ticket consider paid award finder services. That will work as a search term on FT or Google.