I understand why you would burn drugs after they are seized: the harm occurs when the drugs are used, which has yet to happen.
However, I am very confused as to why you would burn ivory. The harm occurred when the elephants were killed, and burning it cannot undo that harm.
In fact, it seems selling the ivory would have several benefits. First, it could decrease the prices in the market for ivory, which would create less of an incentive for people to kill more elephants. Second, it could expand the budget of the government agency responsible for fighting the trade, which could deter people from killing more elephants. Third, it could mean the government agency fighting the ivory trade would have an incentive to fight it more effectively, since government agencies love expanding their budgets.
Also, there is no perverse incentive for the agency here, as long as it is strictly prohibited from killing elephants. And remember, the people who killed the elephants obtain no benefit from their seized ivory being sold in the global market.
Or perhaps they just want to avoid creating a legitimate market in ivory?