I know very little about bluetooth firmware beyond using it for apps and such, but what kind of regulatory reasons are there that prevent publishing source code for bluetooth radios?
To avoid intentionally or unintentionally interfering with frequency spectrum it should not be.
The BT blob (or WiFi blob, or general radio blob) often controls low level aspects of the actual RF device. So you can often alter frequencies, power levels and other parameters. This results in the device acting out of spec.
Now one might say that's the fault of the person doing the modifications/manipulations, but regulations in various countries require the device to prevent these manipulations.
(N.B. I'm not in the exact business, but that's my take away from looking into the topic some time ago)