They aren't introducing Diet Horn here, they're trying to poison the supply chain.
The poachers probably aren't big proponents of a verifiable, traceable supply chain. They don't laser etch a serial number in the things after a kill. So you insert these fake horns into the chain and you dis-incentivize the poaching by driving prices down. Plus you get the witchdoctors questioning whether their supplier is selling them real of fake horns, which can lead to trust breakdowns and stop some purchases.
And hey, the smarter witchdoctors know it's all bullshit they're giving a polish to. So they'll secretly purchase completely legal and probably much cheaper fakes straight from the source and keep giving their victims a show.