This may have changed, but for years the apple warranty was "as long as you don't damage anything, you're allowed to open it". I did some research and at least the manual for my machine still states this, but mine is also from 2012

I took a 2011 model in since I just needed a replacement battery due to bulging. The Apple employee was flabbergasted that I had removed the screws and bottom plate saying I had voided the warranty... on a severely out of warranty device. He was insistent that any device that had screws in it would have the warranty voided if those screws were removed and the device opened, even after showing him the manual which instructs the user on how they can open the device to replace the hard drive in the section labeled "Boost your MacBook Pro" starting on page 35. https://manuals.info.apple.com/MANUALS/1000/MA1602/en_US/mac...

They're getting a bit sloppy with the people they hire recently it seems. I had someone from AppleCare basically tell me I had 90 days after a repair to find an issue with it (violating Australian Consumer Law, which gives you 2 years) - mortified staff in-store gave me my refund though.