Originally Posted by ashill

I think the why they care is you can't take advantage of elite benefits when a different number is in the reservation. That makes sense for, say, prohibiting getting a status-based upgrade or seat assignment when a different frequent flyer number is in the reservation, trying to get bag fee waivers based on one elite status and crediting to another program, or maybe if you're taking advantage of special award availability for elites (which isn't often a thing if ever). But for an award reservation in general, you can book it for anyone, so you're not abusing anything by swapping numbers. I really think resisting changing the FF# is a misapplication of this concern, whether it's in the rules or a human mistake.