[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.