Thats pretty weird since all my fresh installs have the option on by default.
Annoyingly on my MBP, since installing El Capitan, the option keeps turning itself on. Now its become a routine to keep checking the option and forcing it off.
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Another big difference is Microsoft kept all the old OS files, and had an easy downgrade option. For someone not so nerdy, downgrading Sierra to El Cap wont be so easy if they upgraded without knowing what they are getting themselves into.
I dont see the logic in what Apple are doing. If someone wanted to click and accept the OS update, surely they can be asked/nagged and then begin the download if they accept. An automatic download of an entire OS and then giving you the option seems backwards.