Concerning Force weapons

I've seen this several times and it is brought up in many tacticas. This is concerning the instant kill function of DH force weapons. Can someone explain to me why our force weapons still inflict instant kills on units that are immune to instant death (e.g. synapse and eternal warrior)?

I understand that our codex says that the model loses all their wounds and not specifically that it causes instant death, but aren't they effectively the same thing?

On a side note: I think that units being immune to instant death (aside from gargantuan creatures) is the most BS thing ever. What irks me even more is that it applies to all weapons, including destructor weapons (Strength D from apocalyspe, auto wounds, instant kill stuff). I think that it is stupid. If Abbadon the Despoiler got hit by a turbo laser (sort of like being hit by a bunker buster bomb) you would kind of get vapourised no matter how tough you are. Immunity to instant death defeats the purpose of instant death in the first place. (yes I know my opinion on this has been different before, but now I'd like to think that I am more level headed)

Right now we benefit from the fact that the wording in our codex says "...kills outright..." Because of this, our force weapons are the only ones that can still slay characters & MC that have the Eternal Warrior rule.

You can be sure that once we get our new codex, this will go away since they will probably standaradize all force weapons to say it causes instant death.

The real kicker is that 'Instant Death' is a special rule and not just a description. Thus anything that prevents 'Instant Death' prevents everything covered by the Instant Death special rule, such as things double toughness, normal force weapons, etc. However, our force weapon does not cause the 'Instant Death" special rule, it merely kills the model outright. because we don't follow that special rule, things preventing that special rule can't stop us. Using normal grammar insta death would be synonymous with slay outright and all that jazz, but because it's a special game rule with specific stipulations and effects, they arent the same in warhammer.

TLDR; If we used normal grammar, we wouldn't ignore insta death, but since insta death is a special rule we don't have, we do ignore it.

I think, Primarch, that it is a good thing to have Eternal Warrior for non-gargantuan creatures, because it helps display their superiority over other creatures, and possibly demonstrates that they are more than mortal, for instance, Abaddon has the guidance of all four of the Dark Gods

Also keep in mind that the truly special individuals would generally have ways of avoiding such weapons, or they would be dead not special :p