I can't find it in the guidelines right now, but it's disallowed by both Apple and Google. I've personally seen Apple reject apps because of that before (not an app that I created though, since I follow the guidelines)
I found it, it's a restriction on "app builders" or "app generators". If I manually build an app for a client, it's fine for me to publish it in my account. Glad I didn't do anything wrong.