This is basically a WL restriction based on jump numbers (after 100 jumps) as the canopy drills are the same for all levels. But really, a 90 degree flat turn is a good skill to have, but don't you think asking someone with 100 jumps to demonstrate it at 50ft is rather Darwinian? The same with an HP landing.
Eventually you're going to need to make a low turn or land with greater than trim speed (an incorrectly executed flat turn, surge from a braked approach, etc. will happen).
You're much less likely to hurt yourself where that's in a controlled setting (where you're not turning to avoid power lines you didn't see) and you're under a relatively large parachute.
The alternative is "not hook turn type people" (to paraphrase incident reports) getting smaller canopies "because their current parachute is boring" or "so they don't go backwards in wind" who get hurt and dead when things go a bit wrong doing something like landing off the airport and they don't have the right tools to save themselves.
Here's a guy who "wasn't a swooper never turned more than 90s base to final" who didn't use a flat turn to get back into the wind and died at a 1.2 pound/square foot wing loading:
It would be reasonable to require a flat turn and front riser approach to clear people to jump something smaller than a student canopy since the situations where those happen can me more likely for inexperienced jumpers (they're in small groups so they exit later and don't yet have enough experience to decide when a spot is too far so out landings or getting back with too little altitude for a turn back into the wind is more likely).
(This post was edited by DrewEckhardt on Mar 13, 2012, 4:50 PM)