Marc,
All spec series are designed to make setting up the car as easy as possible and therefore calls for stock parts to be used. This makes it easier for someone new to the series to know exactly what parts are to be used so they can set up their car equally to the other racers. If all screws were allowed and someone found a screw that had a smooth section on it that had an advantage to the fully threaded screw then there might be an advantage. This is the case with just about any larger or national series I am aware of. People tend to interpret the rules their way to find as Mark Donahue put it “an unfair advantage”. I am not saying you are specifically looking for an advantage but I want everyone to understand the reason for the rules. It’s also much easier for a series manager to scrutineer the cars when they are all using the same parts.