Word will get out that imitation horn powder is on the loose. That it's not good, could make you sick, even. Next thing you know, the demand will switch to WHOLE horn. Besides, they'll spin a yarn that the mystic effects of rhino horn actually dissipate within 60 minutes of grinding (exposure to the air and such) so that it's actually for the best to get the horn whole, grind and imbibe it in one sitting.
Counterfeiting rhino horn powder (essentially keratin) is one thing. Counterfeiting a whole rhino horn (including the shape, colours, and natural impuriies--which are supposedly the "magic ingredients" in rhino horn) would probably get more complicated.