Stupid never had a definition directly associated with mental handicaps like retarded did/does. Its first use in English (in the 1540s) already meant essentially what it means now (lacking ordinary mental quickness).

In that case replace stupid with idiot, moron, or crazy.

No, but in doing so they imply that the mentally handicapped are fundamentally inferior.

It implies the mentally handicapped are mentally handicapped. Nothing more, nothing less.

If it was of that implication, than why isn't it used as in insult towards those who are mentally handicapped? Just as saying to a girl "you throw like a girl" doesn't really happen.

implying that they think being mentally handicapped is fundamentally a bad thing. See my previous point.

being mentally handicapped IS a fundamentally bad thing.

Do you think those are handicapped would like to not be? Do you not think they would chance in an instant if given the choice? There is NOTHING good about being mentally handicapped, and there never will be. (discounting when the handicap is a specialization rather than a handicap.)

What they are saying is "you are acting as if you are mentally handicapped" which is an insult to those who are not. It is not meant as an insult to those who are.

An analog would be to say "you run like a cheetah" or "slow like a turtle". The person isn't insulting the animal, they are making a comparison that leads to be insulting.

"if you were more like a mentally handicapped person you would be a worse person."

Again, you fail to see the disparity.

If I were to go around calling people black if I don't like them, I am likely doing so because I view black people as bad.

If I were to go around calling people retards, I would be doing so because comparing a person to being retarded is insulting when that person is normal. This does not imply a dislike or a hatred for those who are retarded, it implies a comparison of someone to who they do not want to be like.

In type A, you relate a class of people to a bad thing. In class B, you relate a class of people to the definition of that class of people.

Just as when you tell a man, post puberty, that he hits like a girl. You are saying the man is weak. This does not imply any dislike of women, and it doesn't imply that the person views women as inferior-in-general. It is an insult-by-comparison that doesn't apply to the groups who are a part of that definition.

Now, if you started using "girl" as in insult in any context, for anyone you don't like, then you fall into insult-zones. To be fair though, a decent amount of the use of the word retarded does fall into this category. However, I don't see that being a big issue, because my guess is most using the word in this way will stop as they grow up.

And, in the jon-tron example, the word was not being used as a "general insult" in that way.