SASless - you're confusing design intent with design criteria. Just because Sikorsky designed their S-76 windscreens for bird strike resistance doesn't mean that the bird strike criteria applied to the S-76. That criteria had not been accepted in the US when the S-76 was designed and certificated.
My '65 Mustang did not have rear seat seatbelts. They weren't required when the car was delivered. The fact that they were required in later model years did not mean that Ford had to retroactively install them in their '65 models.
Proving that PHI knew that the cast acrylic windscreens did not meet the design criteria that the S-76 was NOT required to comply with and get a judgement against them for installing an FAA approved part is a stretch. The cast acrylic windscreens were allowed by the regulator. PHI won't be held to a higher standard than the FAA.
A better question, and one for another thread, is why critical design criteria are not applied retroactively to earlier designs.