I often ask these precise questions to determine how sophisticated an options trader I am talking to. I have others that I have asked on this very forum with surprising results. (If they give a better answer than I can, then I know I am in trouble!)
Who bears the risk?
If the stock goes down to zero and you keep your covered call premium you have paid the ultimate price for risk. Broker is happy, you are unhappy.
If you sell a naked put and can't pay, then the broker carries the risk ( they must make good on your obligation ( unless they use something even more nefarious like Reg SHO and FTDs)). Broker is unhappy, you are happy. Police are called, if they find you then broker is happy and you are unhappy.
Therefore to prevent this loophole in the money flow system, they talk down naked puts and invent margin. Or sometimes photoshop bank statements even - apparently the regulators don't clue in for years.
Of course, if you are a bank or a sanctioned broker ( PFG Best, Refco, MF Global etc). the government will bail you out. Which is you again by another name. Broker is happy, you are unhappy.
Do you see a pattern here? Re-use this pattern on other regulations like insider trading, HFT, taxation etc. It helps you understand the money flow of markets -
http://foundingfather1776.wordpress.com/2008/03/19/the-man-and-the-monkeys-a-wall-street-fable/