To keep everything tight and in place even with all the pressure being exerted on it from twisting and pedaling. If you just used the cage mount screws, chances are they'd work themselves out and you'd be left with not only a loose strap, but also a loose pedal cage.
+1. Also - the market for power grips compared to ANY clipless pedal has to be zilch, so I doubt you've stumbled upon a conspiracy for power grips to steal a cold $8 (or whatever) from customers.
Using your same logic, I could argue that Surly is ripping me off because they put
2 sets of eyelets on my touring frame when
one set would mount both fenders and a rear rack -- because they have a secret deal worked out with hardware manufacturers to get people to purchase an extra set of metric bolts for $0.45 ea.