User blog comment:Madman97/You know what really grinds my Wabbajack's? S3E1: I can't kill these guys/@comment-25109952-20131025182427/@comment-13446185-20131029033731

Draevan you're argument is not entirely correct. NPC's can be set to "protected" which makes them player killable only (ex: spouses are set to "protected").

This is definitely the best choice to go with on NPC's. It gives the player more freedom, and if the NPC dies it is their fault.

Bethesda had the choice to put this in but instead made all important NPC's essential. So yes, the decision was made for casual-gamers.