[vesrah]

How does it prevent it? Returning a core isn't required. If you want to stockpile you just need to pay a bit more.

[ATsch]

You need to know the serial number of the phone before ordering the components. That means repairs will for most people take two to three visits to the repair center over a week, whereas apple can do it in one visit.

They know this makes independent repair uncompetitive, which is why they do it.