Penney’s executives may like the chain’s everyday low prices strategy, but investors are panicking. The stock fell 13 percent, the S&P 500’s biggest decline, to its lowest since March 2009, during the recession. Standard & Poor’s Ratings moved Friday to cut Penney’s credit rating — on the same day the company posted its third straight quarter of big losses. Penney unveiled the pricing plan in January; it hasn’t taken off. But the stock has lost nearly half of its value this year.