According to the book it was not “evidently with direct radio transmissions” but as a matter of policy. The bomber crews were given these instructions as a matter of course throughout the war. The only link I can provide is the book itself. Take what you will from it.
Okay, I get it now. I haven’t read the book. But this is the first I’ve heard of the crews being instructed this way, and it struck me as very strange. That whole theater of war (the Balkans) was a little strange by Western standards.