Not so. In the US, the top person at US Nationals gets the top spot at Skate America, where they are feted as the US reigning champion.
Mirai got it the year she was US Champion (2008).
Now if your name was Rudy Galindo ...you have a point. In 1996, Rudy was US Champion and world bronze medallist in his first try at Worlds--and he didn't get Skate America, which he says is why he retired, in part. Of course, Todd Eldredge was world champion but not US champion in the 1996 season, but that was still the custom that the champion got the spot however he finished at worlds
The second & third slots at US Nationals are usually reserved for skaters who did not do well enough at Worlds to get one or 2 slots somewhere else. or are exciting up and comers, making their senior debut (think Gracie Gold this year) Rachael got Skate America in 2009 GP season because she was the reigning Junior World Champion, and got the "up & coming" slot.
In 2010 she got Skate America, because she was the reigning US champion.
This year she got it because she needed it & and because the rules changed so that up and comers who are of the correct age for Jr Worlds, who think they can make Jr. Worlds team but not the senior Worlds team, have to compete on the JGP, not the GP. This includes all girls too young for Senior Worlds in any case.
All the US seniors ahead of Rachael already have 2 slots. Mirai already has one slot. The girls down to 12th are all on the JGP. Consequently, the choice is, does Mirai get a 2nd slot, or does Rachael get a first one?
And because she didn't earn a world or 4CC's berth last year and needs the TES values to qualify for worlds, if in fact, she (very unlikely) made the US Worlds team this year, Rachael got the slot. If she isn't ready to go, USFS will probably give Mirai Rachael's spot, but that would be several a week or two down the road when that choice would need to be made.