Why is the Runesmith enforcing so hard the usage of weapons and there is not much space for free-hand builds?

Many runes already require you to place the rune in a weapon, but then it further enforces you even more to use weapons by feats like Engraving Strike and Engraving Maneuver. Is there an inherent advantage that being free-hand has over wielding weapons that Paizo made it a point to HAVE to use a weapon for feats like this? I am already doing less potential damage by not choosing a weapon, why punish those kind of build more?