I cannot answer as to whether it is by design or not but it doesn't work that way. When you create a PR you are asking to merge all the commits you've made and therefore the comments are related to the PR. These indicate action items the requestor must respond to and only when all comments are resolved is the push allowed.
I don't really see the benefit of having comments on a commit except in the case of a requestor committing a change and asking for someone to review it. However the change is already made so it isn't useful as code approval but perhaps as code review.
Another issue with commit-level comments is that a later commit may invalidate an earlier comment. For a PR with lots of commits then you'd have to look at the final version to see if the comment on a commit is even relevant anymore.
If you have a need for this behavior then you'll need to submit a feature request to have it work that way.