Earlier quoted context omitted.
curl sh is no more dangerous than running any script you haven't read, or binary you haven't decompiled for that matter. The odds of mitm are exactly the same.
There are 2 types of "any script you haven't read, or binary you haven't decompiled": 1. scripts and binaries you obtained from somebody you trust in a safe authenticated way; 2. everything else. When I update my Debian distro, I get binaries via HTTP, that's true. But I also have the public keys of my distro's maintainers whom I trust. The authenticity of every binary I download is automatically checked using those…
Ok, let's say you don't do any of that. The Debian developers are still fallible, and they certainly don't audit the source code of everything they package. Sure, you have to trust something, and I'll certainly agree that it's more reasonable to trust the Debian packagers (and their distribution infrastructure) than a lot of other things, but saying "curl | sh" (when you're curl'ing over https at least, from a website you can reasonably expect not to be compromised) is always bad is a bit extremist. At any rate, the shell script you download is there for you to inspect before running, and if you're not happy with it, you don't have to run it.
Personally, my main beef with that sort of thing is that I want to know if the script is going to accidentally clobber any existing files, or install things to a location where I might not prefer them to be installed.
Edit: I'm only now realizing that the OP has you curl from a non-TLS webserver, which I imagine is the first thing that got you upset. Ugh. Asking people to install something that way is just irresponsible.