San Onofre nuclear plant can only withstand a 7-point quake

When the facility was being built 42 years ago, experts said that only a 6.5 shaker could strike the plant. So just to be on the safe side, Southern California Edison jacked up the maximum to a 7-point earthquake. From the LAT:

San Onofre's three domed units were built in layered shells, like Russian nesting dolls. The outer shell is made of reinforced concrete that is four-feet thick, and is designed to capture any unexpected release of radiation. The inner steel casing housing the reactor is 8 inches thick. Inside the reactor, fuel rods and control rods that make up the nuclear core are surrounded by pressurized water.

But, er, didn't Japan just experience an 8.9 quake? And hasn't it become clear over the past few days that the so-called experts in nuclear energy are not always right? So why wasn't San Onofre built to withstand a more powerful quake?