It's probably pervasive because of github and pull requests where, at least for open-source projects and external contributors, it's true in practice though not enforced.
If you mean the idea in the GGP's post that branches represent a user's work, again I think that conflates repositories with branches. It's not exactly impossible to find people doing work on master of their fork against an OSS project, but best practices (and common practice among most regular contributors I've seen) seem to angle towards using feature branches even then (and asking with a PR to merge eg. stormbrew/blah:fix-the-thing to upstream/blah:master).
Even in that case, you're talking about a clone of the repository, not a "user's branch." That would be the "user's repository", which has branches corresponding to features, and bugfixes (unless they are a user on Github who isn't too familiar with Git, in which case they'll just commit to their "master" branch probably). If each user has their own fork, as is typical on Github, then it makes even less sense to have a "user" branch.
