I can track down the actualy post if you want I don't recall there being a specific announcment, but the logic goes something like this most people who use proxy are referring to fake magic cards printed to look like real ones and therefore talking about proxies how to make copies etc will lead to people cheating or stealing.

For what its worth i used the term proxy for a decade almost exclusively to mean a cheap paper copy of a magic card (or even just writing the word on a sheet of paper and sticking it in a sleeve over a land) and I find this restriction wrong-headed.