Currently it's possible to mount PU scope on SKS rifle, however I'm pretty sure such configuration was never used during Vietnam War (see Additional Information below).

Removing scope from SKS may be seen as "nerfing" the NLF/VC and PAVN, however VC still has access to US M1 Carbine, which can accept scope too (or in fact, any scoped weapon from the DLC that mission-maker would like to use). Also it would highlight the (real life) difference between "hi-tech" US armed forces, and "primitive" weaponry of US opponents.