Mmm. A couple of possibilities:
By using glue they can shave the odd millimetre from the size of the machine.
If you use screws then people will either attempt to replace the battery themselves or take it to some back street dealer and have it done cheap, which can lead to this:
http://www.atsb.gov.au/media/3602097/ao-2011-149_final.pdf
I like these reasons
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If only company X is able to recycle their own products then what happens if company X ends up going out of business (rather unlikely in this case), decides to stop recycling their old products, or changes their recycling process into something worse?
I'm not too worried of Apple going bankrupt or abandoning the recycling program for this generation of products.
What happens if a product from company X ends up at a recycling plant that can't disassemble those products?
The person who gets the product can then bring it home, send it to Apple, and get a gift card. Sounds like a lucky day to me.