Sure, but in practise what are you going to do about it? Have you really never made a mistake in your own flying? Where are you going to find all these infallible instructors?
Now it may be that this instructor was part of a slapdash outfit and was an accident waiting to happen. It may equally be that he was conscientious and for some reason got caught out this one time. I have no idea.
Hang gliding is about as dangerous as general aviation which is about as dangerous as motorcycling so whilst I see your point about taking responsibility for passengers, realistically it is never going to be as safe as riding a rollercoaster or Easyjet and in practice people hang-gliding on holiday are taking on a degree of risk that is likely to be extraordinary for them, as are your passengers if you ever take non-pilots up flying.
Most aviation accidents are due to pilot error of one sort or another. The problem with this error is that it is particularly stark and even a layman can understand it. That's not the same as saying that it is so stupid that no generally competent pilot could make it. Part of the problem with clipping in is that you often have to clip out and in again - often at the last minute e.g. to adjust glider position or wait out a squall; the sort of thing that breaks checklists.
What's the solution? I would wonder about having a mandatory launch assistant to help with the pre take-off checks. Or perhaps even giving the passengers a laminate card with some pre take-off checks for them to take part in. Or put a big red ribbon on the carabiners and give the passenger an instruction to hold it in their right hand whenever it is not attached to the glider. Nothing will be foolproof, but perhaps there is some measure that might reduce this fairly common problem which has been improved but not eliminated by checklists.
This article about clever people making stupid mistakes is one I often think back to.