Stocks under $4 can and are shorted by retail traders. It's costly, but is done on a regular basis. Mainly small brokerage houses will allow you to short any stock under $4 if you have a minimum of 2.50 to 5 dollars per dollar of shorted stock. AND several may require that you borrow from your own inventory as well. NOW, many people elude to large brokerages shorting pennies, which they may do in small doses to maintain market liquidity, but the fact remains that they are not very lucrative for large financial firms and cannot sustain the level of shorting these groups would need to embark on to make a worth wile return. I recommend you do a little research before blurting out that someone's thinking is "retarded"
A very high level overview is below. I'd recommend talking to Zecco or other boutique shops to learn more.