While I agree it was a bit over the top to ground them, it was based on an issue which had appeared in a number of C441 airframes. (fatigue cracks in the construction if I remember correctly). They would most likely apply a similar rule to king airs if it turned out that around X hours a problem became relatively common on them. It was a type specific issue, not a general turboprop problem.\
PLovett, I had a similar understanding, that Cessna said "We cannot guarantee beyond 22000? hours" and CASA said "In that case, they're grounded at 22000"