Why do turkey tail mushroom experts say to tear up the turkey tail, and not powder it?
We have a few Q/As about cinnamon in liquids ([1], [2] et al.) that can help answer your question:

If you grind the mushrooms to a powder, it will form a sludge in hot water that is nearly impossible to filter out and likely unwanted in a “tea”. Without personal experience I can not say for sure (but strongly suspect) that the flavor profile is different than from small pieces - but the phenomenon is known for example from black tea, where different leaf / cut sizes or amount of “dust” matter. From what I heard of turkey tail mushroom tea, it’s rather bitter and that’s likely even more so if cut super finely or powdered. If the majority of sources recommend tearing them up, it’s probably the best balance of extraction, flavor and texture. But you can always run a few experiments yourselves and decide according to your taste and preferences. 

If you want the “full” mushroom compounds, you could look into other preparations, e.g. powder in capsules, but that’s way outside this site’s scope.