It is probably reasonable for average end-user to expect that landmark based search works without enabling the extra setting.
They have option to disble if they care.
They have option to disble if they care.
"Computer, I command thee to send this and only this information over the channel of my choosing, using following encryption scheme, for here be my seal of approval for anyone who might want to verify, and here be the key"
"Sicut Vult"
On Apple's scale, most people care about the things they can do, not about how it happens. For that reason, default matters when the option is only about the internal process pipeline and privacy.
As a result, it is enough to showcase that in case some expert investigates the matter, they show that privacy is considered in a reasonable level.
Maybe some day in the future these things are common knowledge, but I fear that the knowledge gap just increases.