Mystery solved then. Quite possible what Copa will pay vs. what United wants is not lining up whereas with AC or other Star Alliance they will (or some other quid pro quo like even-steven with bought vs. sold).
These airline programs buy the flights on other airlines with some sort of moolah or the best airline will just fill up with people that never have and never will pay a peseta to fly on them. If it were literally an "alliance" everyone has access to every award seat all the time.
This is not uncommon with a lot of airlines lately including OneWorld, AA can't book this but Qantas can, BA can book on BA but CX members cannot, on it goes.
It's hardly egalitarian the whole "alliance" yadda-yadda-yadda.
Even on AA where most of my miles are I just book with them in anytime J class and don't even look for their own saver awards. If they exist (e.g. never) there are 4 connections with one overnight.