Spirit wasn't asking for a government subsidy to get saved from bakruptcy. They were asking to be allowed to get merged with JetBlue (which could've saved them from bankruptcy) and got denied by the government. Those two things aren't the same.
This doesn't seem to be a antitrust issue at all, it looks like it was one company bailing out another.
https://www.justice.gov/archives/opa/pr/justice-department-s...
“Our win in court is a victory for U.S. travelers who deserve lower prices and better choices,” said Assistant Attorney General Jonathan Kanter of the Justice Department’s Antitrust Division. “We fought this case to protect consumers who, as the court recognized, ‘otherwise would have no voice.’ I am incredibly proud of the Antitrust Division’s team and our state law enforcement partners’ tireless advocacy.”