People do have the freedom to use whichever word, for the most part, depending on context. I'm not trying to pass a law about this. I'm making a request in the name of human decency. There's no legal coercion or threat of force going on here.
You don't know what abilities/disabilities people on this board have, nor do you know if they care for someone with disabilities.
There's really no good reason why it needs to be said as an insult, ever, actually. Nothing is "just a word," either. Otherwise, you wouldn't be fighting so hard for the "right" (that no one is trying to deny you) to use it.
Actually, no, I don't.
In a truly egalitarian society, people would inherently recognize that human dignity precludes using someone's disability as a way to insult them or someone else, actually. I don't now what kind of utopia you're envisioning where people go around happily calling each other the N word and R word for the hell of it, but that's not the good world I have in mind.
Now that we're on the subject, you mention "idiot." "Idiot" is an interesting example of language shift. That word has changed meaning over the decades/centuries. Originally, it was used to refer to people with intellectual disabilities, almost in a scientific sense. Doctors and legislators would talk about "idiots" in a technical sense. Between the 19th and 20th centuries "idiot" became more of a slur because people used it that way. So the medical establishment came up with a more "progressive" word, something more technical. "Mental retardation" became the preferred polite and technical way to talk about disabilities. Over time, that term was also co-opted as an insult. Later in the 20th century and through today, "intellectual disability" became the preferred nomenclature. Maybe decades from now the word "retard" will become as disconnected from disabilities as the word "idiot" has. We're not there, though, which is why I ask people (not legislate) that they consider not using that word.