because to some this stockpiling reeks of manipulation of the price by keeping a major amount of monthly supply off the market. And snce Brent and WTI tend to trade largely in tandem, the answer to "why not" is because millions of consumers would end up paying far more at the pump than if Glencore was not choking supply just to boost its own earnings.
http://www.zerohedge.com/news/2016-...-swiss-trader-holds-over-30-june-brent-supply
http://www.zerohedge.com/news/2016-...-swiss-trader-holds-over-30-june-brent-supply