Why can you run a Kotlin coroutine on the main thread

Question

I don't understand how this piece of code can function correctly:

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)

    launch(Dispatchers.Main) {
        log("A")
    }

    log("B")
}

This should output B first, followed by A.

Does this work give that the main thread is already under coroutine control? Maybe code is magically injected into the main thread by the coroutines API.