Arbitrary? Sure. Useful? Hell yeah.
Wanikani needed a way to distinguish between reviewing vocabulary (standalone words) and Japanese characters (kanji). They employed two solutions:
- explicitly telling you which one they’re looking for - kanji or vocabulary (as you mentioned yourself)
- and an additional (arbitrary) visual cue, i.e. distinct colours, that you can get used to over time so that eventually doing reviews becomes almost instinctive
It really doesn’t take that much work to familiarize oneself with the system… I think most of these complaints stem from the misunderstanding of the difference between kanji and vocab (including single-kanji vocab).

