Why is my Bluetooth device showing up as a USBpcap interface?

edit

Devices such as Bluetooth headphones are being searched for as 'Attached USB Devices'.

The "Attached USB Devices" is a list of all the USB devices that USBpcap found on the particular bus it scanned; it's not a list of USBpcap interfaces.

The USBpcap interfaces show up in the list of interfaces on which you can capture; that list is in the main Wireshark window when you open the Wireshark application.

If your machine's Bluetooth adapter is either an external USB device or is internally attached to your CPU over USB, then it will show up as an "Attached USB Device", just as USB keyboards, USB mice, USB Ethernet adapters, USB Wi-Fi adapters, USB disk drives, USB flash drives, USB digital microscopes, etc. will show up in the list of "Attached USB Devices" for one of the USBpcap interfaces.

Those are not, however, interfaces on which Wireshark can capture. USBpcap1, etc. are interfaces on which Wireshark can capture; if you capture on USBpcap devices, you can choose the devices to and from which you want to see USB traffic, and that's what the "Attached USB Devices" checkboxes are for.

However, I'm surprised that Bluetooth devices that are seen by the adapter show up in that list, as they are not USB devices, they're just devices that happen to be seen by a device that happens to be attached over USB. Perhaps USB Bluetooth adapters will, in response to a USB device query, provide a list of Bluetooth devices they currently see, in such a fashion that the USBpcap software will show them as "Attached USB Devices", although that seems a bit peculiar.

What is the difference between the interface called bluetooth network connection?

"Bluetooth Network Connection" is an interface on which you can capture. It might, however, only show IP network traffic over Bluetooth, not raw Bluetooth traffic.