Here's the problem: when I share something on Reader, I know that it will only be seen on Reader by my other friends who have specifically opted-in to say "I want to see stuff Nathan has shared". It is very reasonable to add a feature allowing me to comment on these shared items, since that's a pretty big value add.
When I share something to Plus, now it shows up to people who are on Plus. They are not in "let's read my RSS feeds" mode, they're in "let's see what my friends are up to" mode. My shared article, instead of being just another item to read and potentially comment on, has intruded on their social activity. If they wanted to read about quantum tunneling, they would have gone to Reader, not to Plus. Plus is where you go to read about your cousin's new haircut.