They fined Citadel for corrupting the system built to catch them cheating, and then Citadel sued the system and won.

Here’s the timeline:
2015-2024: Citadel mis marked short sales and misreported 42 billion trades to the surveillance system.
Total fines: $8m on $20b annual revenue.
July 2025: Citadel wins a federal appeal, court declares the surveillance system’s funding unlawful.
January 2026: Citadel files another lawsuit to freeze the rest of the funding, timed so the system runs out of money by August.
Without funding there’s no surveillance.
So it’s not just Citadel that benefits. Jane Street is also facing a federal lawsuit.
The CAT is the tool prosecutors would use to reconstruct evidence; so if it dies before that case reaches discovery, Jane Street’s legal exposure gets significantly harder to prove.
Citadel processes nearly half of every stock trade made and they just dismantled the only tool regulators have to watch what they, and other firms do with your money.