Ziegler can't earn World Standings points at Nationals, but she can earn them at Four Continents. Maybe that's what matters more to her right now.
I think that is exactly what's going on. Team Ziegler is playing the long game and looking ahead to next season. I'm sure this decision was made after 1) looking at the Entries List for 4CC and 2) consulting with USFS.
Entries: Looking at potential competitors to Ava, I see maybe 8: Schizas, Chiba, Mihara, Watanabe, Kim, Lee, Glenn, Thorngren. IMO, Watanabe, Kim, Thorngren are the main threats, assuming Ava herself doesn't implode. Schizas is having issues this season, Chiba can't seem to get it together, Mihara is depleted, just not herself this season, Lee (like Mihara) was already beaten by Ava at NHK, and Glenn is unreliable especially her FS. Team Ziegler has probably done the math and figures that worst case, a 4th place at FCC is worth 612 points. Even if she got to Worlds, she'd have to place 7th or better to get at least the equivalent amount of points...and she knows that isn't likely to happen. A win at 4CC--tough but possible--is worth 840 points, worth better than a 5th place finish at Worlds.
USFS: Has undoubtedly looked at the international scores of all the US ladies, and who is trending up (Ziegler, Thorngren), vs who is going sideways or even down (everybody else in contention for a Nationals medal). Might also have discussed (warned) Ziegler that skipping Nationals is a black mark against Worlds selection but if she was OK with potentially being skipped over this year, so be it. I'd be pretty sure that USFS wouldn't commit to holding a Worlds' spot as TBA until after FCC was over. But they could if they wanted to.
It's a gamble for Ziegler, but in her shoes, I'd be tempted to do the same thing. It's an interesting strategy, let's see if she can produce. A win at FCC or even a medal, would definitely get her further up the World Standings ladder and in better position for next season.