Why is Bluetooth tethering disabled in a lot of Android devices? What is the current state of Bluetooth tethering on Android devices? I have a couple of phones Android 5 and Android 6 that I can use from PCs for Bluetooth tethering. Both phones have support for WiFi tethering also. 

As far as I can tell there are two ways of tethering from the PC. One is to use the old RFCOMM interface where a PPP channel over a Bluetooth serial link is created, where AT commands are sent and where the PC needs to know the APN of the carrier that is in the phone. The IP address is assigned to the tethered device and the phone cannot use that some connection. I have tried this with both phones and this works from the PCs. That was early 90's to 00's technology. I usually use the packet based Ethernet emulation (bnep) and access the Network Access Protocol/Protocol that runs over bnep. The NAP runs over PANU and has and ID of 0x1116. This creates a packet interface of IP packets with and address provided by the phone (with network access) and this is somehow used (using perhaps NAT as in the WiFi case) over the route-able Internet and both devices can use this channel.

I have a couple of tablets that I can successfully tether using WiFi. But I cannot get tethering using Bluetooth NAP. I have searched far and wide and it seems that the only solution for Android is to run an app that creates two new PANU profiles over which a VPN connection is made (because VPNs can be created without root). 

Is my understanding correct? If so why is it that Fire/Nexus/Other tablets disable Bluetooth tethering?
Because some Android phones/tablets ECT sometimes needs a better Wi-Fi connection if that isn't the case then the Android is either not picking up the signal or...
Your Android s not compatible with that Bluetooth