As with most violent rages in my experience, the reason behind it is simple: "AJAX" means Javascript just like DHTML meant Javascript. Why use a new term for the same thing? Why obscure what AJAX really is? Why force my brain to mentally translate AJAX to Javascript each time, and also force me to explain the same to managers that read rather sensationalist tech mags?
My inner angry midget whispers that the reason everybody says "AJAX" and not "Javascript" is deceptive marketing, a well-known root of myriad evils. Remember when the prevailing opinion among serious enterprise-y types was that Javascript should be condemned for enabling annoying browser tricks? Popups, window resizes, and obnoxious uses of the status bar? Eh? Not to mention rampant cross-browser incompatibilities.
The situation is better now. Even if browsers still have important implementation differences, at least each one supports a reasonable, standard-based core of impressive features. For years Javascript has gotten more respect as a "real" language. It even has crashed through the browser walls and onto Java. Javascript was a naughty, naughty brat once, but not any more. Call it by its real name, for Bob's sake! I utterly despise when language is perverted for the sake of rank marketing. Words should communicate, not mislead. Doubleplusgood political correctness can go take a flying leap as well.
Postscript: OK, I admit AJAX is not synonymous with Javascript, so the term does communicate additional meaning. After all, if it only stood for Javascript, it would be "J", right? I guess I wish that AJAX had gone by a different name that emphasized the central tech involved, Javascript. Perhaps one could go X-treme and call it JavascriptX? Or JavascriptRPC? I confess that AJAX is an efficient, memorable way to get across the idea of "browser Javascript getting data without a page refresh by retrieving XML". As I explained, "AJAX" doesn't bug me nearly as much as the fact that the term is deployed daily, with no sense of contradiction, as the next Answer to a Rich Web by the same folks who maligned Javascript.