As quoted from the linked post.
It looks like you’re part of one of our experiments. The logged-in mobile web experience is currently unavailable for a portion of users. To access the site you can log on via desktop, the mobile apps, or wait for the experiment to conclude.
This is separate from the API issue. The are actually going to BLOCK you from even viewing reddit on your phone without using the official app.
Archive.org link in case the post is removed.
I think many people only know what it is because MS Word would (does?) suggest rewriting passive voice into active.
Its a commonly taught thing in English classes where I’m from (Ontario) – we would get harped on it fairly regularly