What I heard was the pipes were 26 year old and designed for 20 year life. The general thinking was that the pipes weren't having any problem with corrosion, but there accident and investigate that followed showed that their idea that should be only minor corrosion over this time period was incorrect.

If they didn't shut the down the operation and any minor or major accident happenned later, they would be at very, very high risk of being sued and not only that but you could easily see possibility of the top management being tossed in jail "for deliberately ignoring this problem". The public in this types of situation could "over react" to send a message, therefore the punishment could far greater than might be reasonable.
Analogy would be similar to launching Shuttle after freezing weather [after Challenger explosion determined to that freezing weather was factor in the accident]. Rest a assured that Shuttle will never launch if weather is freezing, because public opinion is that launching shuttle after freezing weather is same as deliberating destroying the Shuttle and killing the crew. Whether or not the shuttle could be safely launched after freezing isn't really issue, not it's mostly a matter of a public thinking it knows something- whether not the O rings have been redesigned or not isn't really the issue.