This is true of nearly every airline in every alliance, for example it absolutely applies to anyone trying to book United flights using Aeroplan miles.

There's no single answer to this. Sometimes they'll open new partner award space if a flight is selling weakly, or if a route sells so well that new flights get added to it. Again, this is true for most airlines.

If there's a set policy driving that decision process, other than "revenue management determines on a case by case basis" it's not published to my knowledge.
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