No, it wasn't an accident.
In June 2017, the category simply said
"Please add profiles to their unit when it's known.
Otherwise, add the profile to the narrowest category, ie, the first one that meets Regiment, Division, Corps, Army
- 1st Infantry Regiment, United States Army, World War I
- 1st Infantry Division, United States Army, World War I
- American Expeditionary Force (AEF) if served in Europe
- United States Army, World War I if served elsewhere
In August 2017 the language "roll-up category" was added.
On 5 Apr 2019, it was made a top-level, probably because of the numbers of profiles attached. I've been clicking on random profiles and finding a sticker with a units= parameter but no category for the unit has been added. So, making the category a top level is causing people to look at their suggestions to see if there is a lower level category they could use. (And causing several posts to be made in g2g.)
The problem is that for some profiles,the unit isn't going to be known. Maybe the only fact known from something like an obit is that the person served in WWI. In that case, the category needs to be United States Army, World War I.