Tao brings up a great point about origins of diversity and ecological change in North American ecosystems.
Removing indigenous people from their historic lands and land management practices has changed the ecosystems of the Pacific Northwest (and beyond) in profound ways, and I see Scotch broom as part of that unfortunate story
Humans have been the primary force of change in the environment for millennia. Original Americans shaped their ecosystems in ways that created and maintained much of the habitat diversity, sometimes with precision and success, sometimes with blunt tools that could have unintended consequences. They didn't do it because natural living was their preferred lifestyle, they did it to survive, and did so using the tools, plants, and cultural understanding of the world prevalent at the time.
We modern Americans do exactly the same, but in an entirely different context of the tools, plants, and cultural tradition. One of the biggest change in culture between these two traditions, is that we no longer (with exceptions) depend on native plants for food, medicine, or fiber. Instead, native plants are somehow seen as special, or even sacred, which should be looked at but not touched, rather than useful parts of the human economy. Ironic result of the environmental movement.
Is it really any wonder many people don't care about the potential negative impacts of newly introduced species, and focus instead on 'how useful this strange plant from Eurasia could be'?