This is because Hyper-V is a level 1 hypervisor (similar to Xen) while VirtualBox is a level 2 (similar to QEMU). Hyper-V is always running because the Windows "host" OS is actually running under it's control as well.
Wait really? That doesn't seem right, why would they have the host running under part of hyper v when hyper v is optional? This would mean the entire foundation shifts when you enable or disable hyper v
To say nothing of the madness of using it on a laptop with several network adapters when the guest OS only wants to acknowledge one.