Diff Cover Drain

[Storm Trooper]

6.29 Rear axles
6.29.1 Any OEM stock differential housing that was originally offered in an eligible model car is allowed. Updating/Backdating of OEM stock differential housings is allowed.
6.29.2 Both GM and Ford cars may also use a Ford 9”, GM 12 bolt, or Dana 44 rear axle housing, but the housing must maintain both the exact OEM suspension pickup points and OEM rear end geometry as the originally equipped axle assembly.
6.29.3 Any gear ratio equal to or numerically lower than 4.11 that fits the stock/alternate differential case without modification may be used. Differentials may be fully locked (welded) or use any commercially available mechanical limited slip.
6.29.4 Any commercially available replacement type steel or alloy steel axles may be used. Full floater axles are prohibited. Heavy-duty non-”C" Clip style axle ends are allowed and recommended. Competitors should carefully select axle ends. Many of the "C" clip eliminator axle ends are designed for street or drag strip use only and cannot withstand the side loads associated with road racing.
6.29.5 Differentials or differential covers may have a drain plug added.

Can some one explain to my why a aluminum diff. cover in against the rules??