Bluetooth tethering doesn't work

The bluetooth tethering option wich is a normal option keeps a problem in all fyt and non fyt headunits.
I don't understand why it is so difficult to implement that correct. because it is normal when you have a mobile phone with data abbo you have connected already to call handsfree that you can enable tethering on the phone to headunit so the headunit can use the data of the mobile wich is very common accross all over the world.