@PhilBolduc: Good question Phil! This is primarily just a naming convention as you described. We recently added a feature in VSTS whereby you can check a box to delete the topic branch when you complete a PR. (When you click "Complete Pull Request" you'll see it on the next dialog)
While the option to secure a branch/repo exists in VSTS/TFS, we don't typically lock things down. Topic branches are wide open, and there are valid reasons where you might be collaborating with somebody on a change that you might need to go edit their topic branch contents. I can't think of a time when somebody accidentally or maliciously changed somebody else's topic branch contents, but since all these changes are ultimately logged in VSTS/TFS there isn't much of a concern here.
Where we DO enforce security (by way of branch policies) is in the master branch. Note that we don't apply ACL's to the master branch, but instead we rely on the branch policies screen (which I showed in the demo) to define the quality gates whereby changes need to flow into main. This implicitly guarantees security because there are specific reviewers who have to approve changes into main.
Hope that helps!