If swapping out an HDD was similar to replacing their brake system with one you built in your garage, I'd agree with you. Really, it's more like swapping out your headlights. They're designed to be modular.
Not for Macs, seemingly! The point I was really making was one of general 'entitlement' the average person has when they walk to a Genius Bar. So, poke around at your hearts content but don't expect Apple to bail you out if it goes wrong. The caricature to that argument is essentially a hackintosh