OK>..if you have a two engine plane and you lose one engine, regulations in the USA require you to land at the nearest suitable airport.
IF you are in a 4 engine plane and lose one engine, you can fly anywhere you like for as long as you like...BUT common sense is one thing that can't be regulated. I opposed the BA 3 engine journey accross the pond...but it wasn't illegal.
It would seem to me that the place to look in this situation is: had the engine been over temped at any time, causing a weakness in the turbine section? had this set of turbine blades/disc been properly made?
we shall see...that the plane is intact, the passengers are ok are a tribute to the crew.