"They" who? A lot of companies sell DE.
People mixing it with fluid are really silly, as that counteracts any desiccating effect which is supposed to be the main mode of action. Besides the abrading effect from super sharp particles sand papering their insides. Glad you don't do that to...
Birds are birds. A very fine powder becomes airborne so easily that even a careful application will disperse some. It's fine to take your own risks with your own animals, but it might be a good idea to add a disclaimer for a controversial practice many warn against.