I have not seen the Vail manuals. Where would I go if I wanted to see them?
I think Rusty is talking about the "PSIA Children's Alpine Teaching Handbook" and "PSIA Adult Alpine Teaching Handbook", produced with the Vail/Beaver Creek ski school, available here: http://www.thesnowpros.org/PublicationsVideosResources/Resources/AlpineEducationalResources.aspx . Those are very 'nuts and bolts' handbooks with tons of ideas about drills and progressions (among many, many other topics). If he means something else he'll have to clarify.
One other minor quibble (directed to Matthias99): I don't think liability concerns would necessarily preclude ski areas from grading skiiers, and thereby preventing novice skiiers from entering intermediate or expert terrain and causing injury to themselves or others. In fact, I think a case can be made that by failing to evaluate skier ability prior to issuing lift tickets, areas are in fact incurring liability.
My understanding is that if a ski area (or other business in a situation like this) takes a hands-off approach and makes it clear that is what they are doing, they are unlikely to be found liable for injuries resulting from interactions between customers. Everyone who visits is choosing to ski and share the slopes at their own risk -- the ski area just provides access, maintains the lifts, and does avi control and patrolling. This seems to be a common model in other sports.
If a resort starts rating/grading skiers and riders, they are (at least implicitly) promising that all the skiers/riders on the 'advanced' slopes are competent. If someone gets through that is not competent and hurts you, in theory you could go after the resort on the basis that they should have stopped that person.
Would such a lawsuit succeed? I have no idea. In my (non-lawery) opinion making a best effort attempt to improve safety should not make you liable if doing nothing would not have made you liable. But maybe it doesn't actually work that way in practice, or they don't want to risk it.
Someone else mentioned resorts that require extra training before you can use the terrain park; this is somewhat different. The bigger risk in a terrain park is normally falling and breaking yourself, not having a collision with another person. (Although that does happen too.) I'm guessing they do this because whoever provides their liability insurance feels it makes them less likely to be sued by someone who gets hurt in a terrain park, so they pay a lower rate for the insurance.
Disagreeing with PSIA is fine. If you think things aren't working well, you should try to make them better. But implying that PSIA wants their lessons to suck so that ski schools will make more money (somehow?) is nonsensical and not constructive. I've only been doing this for a few years myself, but I can see that a lot of people have put a lot of work into their curriculum, and as mentioned above there is a method to the madness.