I think the lack of control is undesirable. I agree the feature itself can be very useful. The title of this post sounds way more sinister than it appears.

What possible controls can Apple put in place though other than a checkbox for do/do not sync ?

And remember Apple is a positioning iCloud as a cloud for dummies solution so adding choice comes with trade offs.

Couldn't I apply the same argument for while google needs to send your call history to its servers to perform number lookup? Yet people cry fowl (rightfully so) about privacy implications.

Google has a track record of disregarding user privacy. Apple has the opposite, a track record of protecting user privacy as much as they can.