Of course they are. They love meat, and it is great for them. No question.Actually chickens are omnivores... But i won't get into that.
In addition to bugs and small rodents via free range, my layers get BSF. I raise BSF the majority of the year here (for free thank goodness) but I don't feed them all directly. I freeze a percentage of them and feed them over the winter months when we have no bugs to forage.
They are actually included in my winter ration and I weigh them out each morning (yep... there's that anal side of me again) LOL
I never feed BSF to prelaying birds (well... almost never, I do toss them a few as a treat every once in a while). But BSF are way too high in calcium for chicks to eat on a regular basis.
When all other livestock is butchered throughout the year i save all the meat and bone meal... This is what the chicks get.
The remaining, non-animal protein part of the ration is grains we grow or acquire locally that are fermented prior to feeding. I have a hammer mill so I grind them a little finer for brooder chicks who begin on fermented grains from day 1. By the time they are 6-10 weeks old they are eating fermented whole grains like the rest of the flock.