Actually this is only on Google Collections which is a bookmark sharing service, not just the browser bookmark sync.

Your synced bookmarks should be unaffected as Google says everything in browser sync is encrypted locally before being synced, so Google shouldn’t be able to scan those (or really care about them at all)

They are doing it to protect themselves from legal liability specifically because of the public-facing nature of Google Collections bookmarks.

They don’t care about your synced bookmarks or browser history because it’s not public-facing and they aren’t legally liable for the contents