>Cut >50% and it's now a extortion

Extortion (or blackmail) has a legal definition and this isn't it. In California (where Apple is incorporated and headquartered), the statute states:

Extortion is the obtaining of property or other consideration from another, with his or her consent, or the obtaining of an official act of a public officer, induced by a wrongful use of force or fear, or under color of official right.

Wrongful use of fear exists in this context as there is a threat to expel the developer out of Apple ecosystem if he does not accept the cut.