By reducing the number of Parts that they have to carry.
The fewer parts that they carry the cheaper it is for Apple to
"Service" their products. Though it's most certainly not confined to Apple they have taken it to the extreme with the Glued in Battery concept.
I remember recently how Sony Batteries could catch Fire and here you have to believe that Apple isn't making the Batteries but buying them from one of the Big Battery Makers who in the past have had problems.
So if they get a Bad Batch of Batteries just who do you think will pay for the replacement of the Case, M'Board and so on. I can guarantee that it will not be the Battery Suppliers who will be part of the real problem.
I currently have a iPhone 4 to replace the screen on well actually the front glass after a fall. The back is easy 2 screws and it slides off. The front is a different story with a lot of work involved to get it off let alone back together and working again.
As for Techs damaging the units I vehemently disagree! I always find it's the users who do the real damage and here they are within their rights to dismantle devices to attempt a repair as the warranty under our LAW only prevents third Parties from dismantling the devices. If you work for Apple and the user dismantles it you have to carry the costs as they are one of the parties that is allowed to after all it's theirs not Apples.

Sure you can charge for what they may break but if they completely strip the thing and hand it to you you carry the cost of the time to repair and rebuild the thing. I can remember 45 years ago a customer ringing up internationally asking how to secure their product so it wouldn't get damaged in transit tot he Companies Distributer here. I said just bolt it to the case and it will be fine.
What I didn't know is that they had totally reduced it to component parts so they screwed the base to the case and threw the rest into the case before closing it up tight and giving to the airlines to transport.
What was a 15 minute repair to do took well over 12 Hours and we had to eat the costs of the reassembly because under the Warranty only the Supplier and the owner had the right to pull the thing apart. The idiot thing here is if a Qualified Repairer that was not either from the Distributer or it's Authorized Service Agents or the Owner the Warranty is Void even if they do no harm where as a Unqualified Owner in no way affects the Warranty.
Col