[tmottabr]

It is not law..

It is the fact that if the found out you would have the imperial Sardukars (that were basically invincible) plus all the Landsraad houses going after you and your house would be effectively gone. What stopped was the huge risk.

This did not stopped Paul from using it in the Arrakeen attack that we will see in the second movie. If Paul and the Freemen lost then house Atreides would be gone either way so he had more incentive to use it.

The Dune universe favor war of assassins that would happen with Landsraad approval. It is established that Atreides and Harkonnens had many war of assassins between then pre dune novel.

[vitiral]

IIRC they didn't "use" it -- i.e. they didn't intentionally shoot their own shield with their own lasgun. Rather they shielded a few underground areas and the Harkonen were using lasguns, something you're not supposed to do.

I could be misremembering though.

[tmottabr]

I think they did use it intentionally because without that nuclear explosion they could not get rid of the mountains that protected Arrakeen from sand-worms.

They invaded Arrakeen riding sand-worms that would be impossible before, it is even said in the novel that Paul was riding the biggest sand-worm ever seen that day and the Freemen considered this some kind of sign they were going to win..

now i don't remember if they shoot their own shield or they knew the Sardukar would be using lasgun and took advantage of it..

EDIT:

Ok, it seen as i am misremembering things.. They did not used a lasgun\shield combo, they used actual nukes.. Either way consequences and effects are the same..

[vitiral]

Oh I forgot about that.

I thought they were relying on a bit of a loophole: they didn't kill (m)any humans, "just" a natural barrier, which is why they could get away with it.