It depends. Some of the set-asides in the U.S. departments aren't voluntary. There is a government commitment for a 20% set-aside. I can't remember which piece of United States legislation it's in, but I could look it up for you.
A lot of the culture around the set-aside market in the U.S. is just that this is generally a good thing to do. If I had to pick a favourite government department to look at, I would encourage you to look at the rapid adoption of clean energy technology undertaken by the U.S. Department of Defense.
Mind you, they're doing it for reasons other than just a desire for environmental performance. It has to do with energy sovereignty. Still, they have one of the largest research budgets in the world.