Default AirPlane mode turns off cellular and wifi. FAA allows bluetooth accessories, headphones, keyboards, hearing aids, etc during all phases of a commercial flight and with the AirPods, AppleWatch, Beats headphone line up Apple likely felt it was more convenient to leave BT on.
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@Paco II mentioned though if you turn bluetooth off in AirPlane mode the next time you turn on AirPlane mode it will remember that and shut off bluetooth.