You are kind of correct.

Not all brokers will allow it and some only allow it for certain hot penny stocks...ie THCZ lately.

I think overall it's bad practice for the SEC, OTC and other markets to let anyone short sell stocks...leads to crazy volatility.

But the reason its allowed is because it also creates liquidity...if people didn't short stocks and they didn't want to buy them then it would make for a lot of stocks with no liquidity as there would be no volume.