Shorting a currency does not always result in a drop in it's price, if it did everyone would be shorting and getting profit off their investments. I believe short term traders predict a possible drop in the price and take advantage of it rather than causing the drop by selling their holdings.
I am not an expert trader and do not have deep knowledge about the trades and how actions affect the market.
And to it, I think shorting also would generate more liquidity to the brokers or exchanges as the case may be since it involves not staying long in the market or waiting for lots of profit, and then, re-entrying again. In actual fact, it such practice should bring more gains to bitcoin.