I think it is as much about ignorance and stupidity as it is lack of etiquette.
I am confused by the situation described by the OP. Were the skiers who were cutting across the hill a) initially stopped, then started the traverse across the hill or b) skiing downhill, then decided to cut to the side?
I think that makes a difference in terms of judging the situation.
We all get ticked off a bit when someone is in a spot that interrupts our sweet line. Whether they are stopped or moving. Buuut, I hope you noticed those traversing skiers that you were overtaking well in advance. (I assume you were overtaking them. See the two caveats below.) If you didn't notice them, then you need to seriously improve your skills (for safety and better sliding.) Far too many sliders are not looking downhill, but just past their ski/board tip(s).
To me, a dangerous ski situation is when several advanced skiers happen to be skiing parallel to each other on different sides of the trail. It is a lot more difficult to notice someone skiing to the exact side of you (versus in front of you), especially if they happened to start out at the same time as you did.
Kids doing jumps sideways across a hill (see that a lot around here) are a very dangerous situation. They are generally stopped prior to the jump, and they come across the hill sideways into or just below oncoming skiers, often a bit out of control. Very difficult to notice them. Not sure who would be a fault per the skier's code.
Speaking of the skier's code, there is some grey area and possible contradictions in the skier's code. At least in my reading of the code. For instance:
People ahead of you have the right of way. It is your responsibility to avoid them.
Vs:
Whenever starting downhill or merging into a trail, look uphill and yield to others.
And
You must not stop where you obstruct a trail, or are not visible from above.
So, you could have both the uphill and downhill skiers guilty per the skiers code.
There are several common sense items not mentioned in the skier's code that skier's routinely ignore. Don't ski too close to someone when you pass them (if they are stopped or moving.) When passing skiers, expect them to suddenly vary their turn shape. Look uphill when traversing across a hill. When skiing downhill, expect skiers to be traversing across the hill at trail junctions. On a bump run, try to not stop in a prime zipper line.
One more subtle point to the OP. While I'll often keep the same turn radius and turn tempo when skiing down a hill, especially if I'm working on drills, I may decide to suddenly vary it. (The ability to vary your turns like that while keeping in control is important to advanced skiing.) I try not to do this on crowded trails, but as much as possible, I avoid crowded trails and areas.