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The MSM got it right. Defective is the word. The defect has shortened the original projected lifetime of the part.

http://www.freerepublic.com/focus/f-news/1951577/posts

Time compliance technical order inspections have discovered nine other aircraft with longeron fatigue-cracks. Additionally, approximately 40 percent of inspected aircraft have at least one longeron that does not meet blueprint specifications."

Robins team believes last snag cleared in F-15 groundings

A thinning of the longeron at a key stress point - possibly due to a manufacturing defect - may be the root cause of the mishap and the cracks found in the eight aircraft.
"More than likely it is a manufacturing issue and we have pulled all the Boeing material discrepancy reports," the group commander said. "So far, we have not been able to isolate it to a particular production run or series. The cracks show up in aircraft as old as 1978 and as new as 1985."
Recurring checks had not called for a review of the area in the past.
"It was a 25,000 to 100,000-hour part," the colonel stressed. "So it was not included in our depot and phase inspections. It was designed to significantly outlast the aircraft."
However, the Robins team has learned that a number of factors create additional stress on the component - a splice joint in the two-piece longeron with different material thicknesses coming together, reduced width proceeding from the joint, angle variations and changes in the canopy sill.
"So a large amount of activity occurs in that single spot," Niemantsverdriet acknowledged. "It's like a creek that narrows down, making the water flow faster."

The lifetime charts that were up here on FR when this story broke were based upon 250 hours per year of operation. Most the F-15's have far exceeded that. If you look a calendar years may be but in flight time I see they made their numbers.

From one of your links:

A thinning of the longeron at a key stress point - possibly due to a manufacturing defect - may be the root cause of the mishap and the cracks found in the eight aircraft. "More than likely it is a manufacturing issue and we have pulled all the Boeing material discrepancy reports," the group commander said. "So far, we have not been able to isolate it to a particular production run or series. The cracks show up in aircraft as old as 1978 and as new as 1985." Recurring checks had not called for a review of the area in the past. "It was a 25,000 to 100,000-hour part," the colonel stressed. "So it was not included in our depot and phase inspections. It was designed to significantly outlast the aircraft." "

So there maybe a manufacturing defect but only supposition at this point.