I appreciate your concern. However, you are only partially correct. In fact, the feathers that are hugely problematic are the cheap, artificially dyed, poorly secured kind found in boas and factory-made (totally inappropriate) head-dresses. The feathers seen above are part of an intricate costume hand-made with care – each and every, natural feather glued securely by a veteran Burner of nearly 2 decades.
I've been attending Burning Man since 2000 and, I assure you, I genuinely care about the 10 principles, especially leave no trace. So does the artist who created these costumes, as well as the women who dance in them – one of whom has been attending for 22 years. The Org is well aware of these costumes – they don't object to their presence on-playa.
I've been photographing the ceremonial dances done in these costumes since 2008 – I've paid close attention and not once have I seen them shed even the tiniest downy feather.