When Spirit bought the P&W engines they did not come with unlimited warranties. Airlines can purchase different levels of warranty on the engines. The big airlines always purchase almost every type of follow-on insurance they can to mitigate risk.

Spirit chose not to.

You can't pass on optional maintenance packages because you want to save costs and then when the product has problems, blame the manufacturer. P&W gave them a gift by offering them credits toward repairs and inspections in lieu of litigation but those credits cover only a fraction of the costs.