It does seem to come up an awful lot, but I’m not sure editing the FAQ/rules is a solution (FAQ - Anki Forums). Doesn’t that seem like it’s a long way from where the issue is happening? There are cues right on the page that folks are missing.
We address this in the About/pinned post in the Anki parent category, but not in the AnkiMobile and AnkiDroid parent categories, so that could be updated.
I don’t know if that “you’re not allowed …” message is customizable, but I agree that an update to that could help too.
[I edited your title because I didn’t understand the relevance of the “15 characters” part.]
You are right about the cues, but often, people don’t look into it as much as they should and then subsequently miss them.
Also, what all count as top-level categories?