The shelves have a keyhole that slots over an exposed screw head. In solid walls this is fine - you just tighten the screws up until the right amount of shaft is exposed and drop the shelf over them.
Fixing to plasterboard is another problem. Most plasterboard fixings rely on the screw being tightened to clamp the fixing from behind. This is not feasible as the screw needs to be left protruding.
The only type of fixings that I have used that do not do this are the little die-cast screws with a large thread that screw into the wall first and then screw the actual screw into them.
I am not convinced though that these will not pull through with the forces placed by the shelf. There is nothing behind the plasterboard to spread the load.
I there any fixings that I have missed that combine something behind the plasterboard to spread the load with the ability to hold fast when the screw is not fully tightened?