yes they can, the reason you cant... is because
you are NOT an Accredited Investor, Market Maker, or Hedge Fund
regular john doe retail investors cant do it with regular discount brokers, as opposed to full-service brokerages, unless they have accounts with brokers to do it, like suretrader, Interactive Brokers, or others offshore
and even then there are many restrictions, limitations and collateral requirements, which dont make it feasible to do anyways
ASCM is a perfect example, they are prolific shorters in the OTC market
Market Data 
Markets 
