In reply to the sheep:
I've used lots of energy powders in Camelbak and Platypus bladders without issue.
Unless there is something specific about yours, the only issue I can think of is one of hygiene. Perhaps the manufacturer has taken a look at the biological warfare breeding ground that is the inside the typical rehydration bladder drinking tubes and wants to have no responsibility for the mutant life forms that might emerge from one of these someday.