Whoever wanted the PCs to be rubes?
I don't think of it as being discouraged from using their own ideas so much as how they come up with the ideas. What is the impetus for the GM's new idea? Is it related to the PCs in some way? This can be specific (a relative or an enemy or similar) or can be broad (a danger to X when one of the PCs is sworn to defend X).
It's just a different way to channel your creativity. We've talked about this kind of stuff before. You actively don't want the game to be "about" the PCs. They're all interchangeable in your eyes (or very close to that). These games are inherently about the PCs. So everything the GM does is meant to be in service to that idea.
I think that shift happened much earlier than 4e. I'd say mid-90s, although I think there was plenty of that stuff sooner. At most, that was a phase that you were meant to endure and move past as quickly as possible. And there's a reason... it stinks.
Over ten years of staring every game that way was enough for me. I don't need to try and control the power level of the PCs just like I don't need to limit their knowledge to of runic circles or what's over the next hill.