Why not? Margin trading is there on Poloniex which allows you to short over few selected coins and make money.
You can do leverage trading that allows you up to 2.5x leverage over your capital which is an advantage, though if market moves against you even those short sells could make you lose a big amount of your capital in a matter of minutes. When they short, they actually help the dump in a different way by putting their money on the sell side which increases the selling pressure over buyers.