The simple answer is that there is a growing movement to reduce user options that can break applications.
Please try to remember whose machine you're running on. You're a guest under my roof, and guests that behave badly do not get invited back. So no, you don't get to run code in my browser until you've earned a certain level of trust, and you certainly don't get to invite in your friends' code. (I mean, just who the fsck is rpxnow.com, anyway?)
The technical term for sites that behave this way is, "Broken."
These programmers are called, "Wrong."
Back in the 1990's -- in the days of sneaker-net, recall -- macros in Microsoft Word documents, originally thought to be oh so terribly clever, proved to be a monumental nightmare for their ability to spread viruses and generally wreak havoc. It was so bad that even Microsoft was forced to admit it fscked up, and no longer executed macros in a loaded document by default, but would ask first. So you'd think the lesson on embedding executable content in what was fundamentally a document would have been learned.