Isn't that a risk that just comes with MTBing on any terrain?
Yes - any waiver will make that clear.
I think the case balances on a combination of factors; whether the feature in question was suitable for a beginner course and the skill level of those being trained & whether the tuition available was of sufficient standard to mitigate risks, and like above, whether the trainee was pushed into riding beyond his ability to the point where he was put at unnecessary risk.
The trail in question - whilst 'designed' - isn't a managed skills centre trail, where the risks can be very closely controlled. It will be interesting to see how this pans out.
FWIW I think the trainee was incredibly unlucky and my heart goes out to all involved. No winners here :(