They didn’t. They said it is incredibly risky for a third party app to have MDM on a customer’s device, for exactly the reasons that enterprises using MDM want it: because it gives them an extraordinary degree of access and control.
The apps being evicted were sold in Apple's store. They went through Apple's review process. Apple made money from their sale.Tsai Blog said:Apple Cracks Down on Screen Time Apps That Use MDM
It’s hard to believe that Apple only recently figured out that these very popular apps had been using MDM for years . . . There’s no evidence presented that any of these developers abused the power of MDM.
If a giant corporation is allowed by Apple to lock down and monitor iPhones assigned to employees, why shouldn't parents be allowed the same "powers" to protect their children? Wouldn't these "parental monitoring apps" have protected kids we recently learned were lured by Facebook and Google into side-loading really bad spyware?