The PW4000/94" engine has been in service for over 30 years with over 150 million flight hours - it's a big, heavily used fleet. The odd turbine failure that spits blades out the back when you have an engine with such a history is generally not that big of deal. Every engine type has the occasional 'metal in the tailpipe' incident - it happens. Sometimes things break.
I haven't had access to the PW4000/94" reliability statistics for over four years, but when I retired it was considerably less than one IFSD/100,000 flight hours standard that's used for extended ETOPS.
Combined with Longtail being a small time freight operator, me thinks this is a pretty dramatic over reaction by the Japanese authorities.