But commits change branches. None of the commits started on the "master" branch, they started on some developer's branch (which might also be called "master" in a different repo, but is still separate).
Say for example, I'm looking at a freshly cloned repo. There's a first commit and most-recent commit on master - I can identify them with git-log. The problem is that I cannot view the path of commits between these two if I'm only interested in the commits made when the current branch was master (which is generally the case unless I want to drill into a feature branch).
Disallowing merges makes the problem go away but that removes a lot of options in terms of work-flow.
When I'm working none of the commits are made when the current branch was master. They are made on branches, where commits are finalised, tested and signed, and then master is fast-forwarded to match the branch tip commit (or something intermediate if I'm satisfied with that). Conflicts are detected by the fast-forward and reconciled locally in the branch, re-tested and signed off again.
It's rare that the current branch is master, and no commits are created while on master.
So what would you like to see?