The purpose of that allowance was to provide SOME relief, in terms of availability, ease, and cost, while constraining the parts to the "equivalent" of OEM parts - to take advantage of the aftermarket parts stream that follows cars around the marketplace.
What you're missing - or more likely don't agree with - is that while that logic could be extended to include plastic windows (etc.), the perceived risk to the stability and consistency of the category is judged to be too high relative to the benefits to individuals likely to result.
"The needs of the many outweigh the needs of the few."
K
Mark 2:27 Then he said to them, “The Sabbath was made for man, not man for the Sabbath.
You sound like a Baptist who wants to outlaw fornication because it might lead to dancing.
What is the philosophy of the category?
Improved Touring classes are intended to provide the membership with the opportunity to compete in low cost cars with limited modifications, suitable for racing competition. To that end, cars will be models, as offered for sale in the United States. They will be prepared to manufacturer’s specifications except for modifications permitted by these rules.
When the price of replacing a fender or bumper or window glass becomes expensive, the rules no longer meet the category purpose -- competition in low cost cars. The equivalent rule made sense when such parts existed. In an increasing number of cases, and probably at an accelerating rate, the availability of such parts is becoming problematic. The purpose allows for limited modifications. The purpose calls for low cost cars. The idea of creep went out the window long ago. Shocks, exhausts, spherical bearings, alternate final drives..... clinging to a prohibition for alternate window material is like someone claiming they are a virgin because it's only been the tip.