People often ask about this restriction. It's worth explaining why.
Basically, in our Public channel, we want the highlights on text to be of value to others. A single highlight by a single user without any other content doesn't meet that threshold in my view.
Certainly, in the Kindle, Amazon will show you "things other people highlighted". That's different in our eyes-- it's a statistical representation of attention. We might consider that at some point ourselves. The question here is: Should we show every single highlight as a separate thing on a page just because someone was making a personal underline for their own benefit. To us, the answer is "no".
We can see the benefit of you being able to personally share a bunch of highlights you've made with a specific person or group of people. That to us is probably a separate permission level, and a separate sharing model. For instance, you might share a specific link to your highlighted version of a document.
TBD.