SK rules are in place firstly to avoid situations where one team dominates spawn area during whole map duration. That can steal the fun and make people disconnect. So this is one worst case scenario that rules prevent.
To sum up SK rules:
Heavy limitations.
Light weapons for shooting back.
Allowed on capturable spawns.
SK areas for detailed info.
So here is the question about knife as it is least efficient weapon to dominate spawn yet u can pull out some sneaky fun plays.
During spawnpassing sometimes u see there "afk" enemy so u have option to continue and hope he really is afk or he will shoot u in few secs. To safely pass i would rather punish this afk by backstab as afk not tolerated in first place. There should not be an afk in spawn in perfect world :D. There is no way to kill person in spawn unless he is shooting you.
I think i have even seen people kicked for knife kill in SK area. As it is by rules every kill in area is SK.
I see more to gain than lose by allowing knife in SK area. Lets see if people agree or disagree