The "Apple Tax" pays for the maintenance of the Apple ecosystem. Epic wants access to this amazing ecosystem, but resents having to help pay to support it.
Either "third party" app stores will also pay to support this ecosystem, in which case Epic will be paying the same amount either way; or they'll fail to support the ecosystem, in which case Apple will be forced to turn to other sources to make revenue (like selling personal information).
Right now I, as the consumer, can choose between an ecosystem whose business model is based on selling personal information (Android) or an ecosystem whose business model is based on the end user paying more (Apple). Don't take that choice away from me.