In this specific case, it's a crappy IoT device supporting only TLS 1.1. So when my browsers "helpfully" redirect me from the http:// interface to the https:// interface, they then "helpfully" refuse to work because TLS 1.1 is icky and end-of-life and browsers have decided that means "tell the user to fuck off."
At the end of the day, I worked around all of this browser helpfulness by using Firefox and re-enabling TLS 1.1 in Firefox's settings. I never actually managed to force any browser to use http:// despite trying many proposed solutions in this thread and on the web.
But all that aside, it doesn't really matter why I want to use unencrypted HTTP. I am commanding my computer to do it, and I expect it to carry out my command. My computer is a tool. It should do what I want it to do, even if that might hurt me. If I type in sudo rm -rf /usr, I expect it to do what I tell it to do. Not ask "Why do you want to do that?"