If you're going to include knives and related challenges into a game where guns are the dominant weapon used, why would you not buff them? If it takes 4 hits to kill someone with anything under the Fällkniven A-1, where a quick trigger finger, or even just running away will prevent you from obtaining your kill.
Also if someone with ammo simply hits you with their weapon, automatic knock back. Then you become a doll for them to beat at their leisure using a gun with no bullets. Bless the inherent lag.
Sincerely,
They guy running for the 5 kills with a melee weapon challenge.