Why can they do so on a mature project? Why can they not only push to their branch, have the repo have commit/push hooks for fast linting, have CI builds for each commit, and only merge into master when comfortable (e.g. after QA)? And if it's a fairly new project and they're working in master directly, what's the harm of broken code?

Even non-idiots make mistakes. Use the pre-push hook that comes with your repo: https://github.com/git/git/blob/master/templates/hooks--pre-...

Then, just mark your commits with WIP.