If you want any more answers to why. then query the FAA, not me, I just read and comply.
I have no idea how you come to the conclusion that a 150 horse O-320-E2D must have the plug removed and inspection performed. There is no evidence to support your claim.
The whole thing won't copy paste so I have snip it but here is the link
http://rgl.faa.gov/Regulatory_and_G...E7B86DE103CB30E78625684D006644B5?OpenDocument
...In addition, the Federal Aviation Administration (FAA) has confirmed that four other failures in the United States and 10 in foreign countries were due to cracks initiating from corrosion pits in the crankshaft bore on certain Textron Lycoming 320 and 360 reciprocating engines with ratings of
160 horsepower or greater.
...These studies and data confirm the crankshaft fracture occurred at a stress concentration caused by a corrosion pit on the inside of the crankshaft. In addition, since the NPRM was issued, six additional
crankshaft failures on 160 horsepower Textron Lycoming engines are being investigated. The FAA has, however, performed additional analysis to limit the population of engines impacted by this proposed AD and has deleted the five year limit on pitted crankshafts undergoing repetitive FPI inspections. These measures will decrease the cost of the AD to the public...
...One commenter states that the AD should take into consideration the operation and service history for each engine in specifying corrective action. The FAA partially concurs. The FAA has taken into consideration service history and has limited the applicability of this AD to
engines with 160 hp or greater....
...The total number of engines impacted worldwide is 16,357 (
11,000, 160 hp, 320 series; and 5,357, 360 Series). The FAA estimates that 60% of that number, 9,814 engines are installed on aircraft of U.S. registry, and are affected by this AD. The FAA estimates that it will take approximately 8 work hours per engine to accomplish...
If that wasn't enough the actual text of the AD written in Title 14 Code of Federal Regulation Part 39.13 amendment 39-10291says