Quoted from CaptainNeo:the mounting had to be unique because of the huge cut in the playfield due to drac. They already had the problem with drac section warping somewhat. It's for the underside hardware. Pops are the same way, just are hidden. Better than if they put T nuts there instead.
I don't understand how fastening through the playfield would prevent that bit warping though. Same weight pulling that section down. Surely all this does is make the connection more secure.
Anyway, mystery solved, thanks. Makes me wonder though. What other games have things like this right in the middle of the playfield? It took me a while to notice this and it's one of my favourite games so maybe there are loads of them?