Why can a subclass override a method by removing the nonisolated keyword?

Using Swift
1

Consider the following code:

@MainActor class A {
    nonisolated func foo() {
    }
}

class B: A {
    override nonisolated func foo() {
    }
}

Why does the compiler allow me to remove the nonisolated keyword in the subclass? The code still compiles, without errors or warnings, even when using the Swift 6 language version.

This is causing me great confusion as the AppKit NSDocument class is marked @MainActor, so I thought that all overridden methods that don't specify nonisolated must also run on the main actor, but then

override func read(from url: URL, ofType typeName: String) throws {
    Swift.print(Thread.isMainThread)
}

prints false.

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