Because the world isn't black and white. There could be soooo many different reasons why you're willing to pay one person a lot more than someone else to do a job.
Interestingly, the architects of these laws would agree with this point.
Forcing them to be upfront about the requirements for more money results in candidates better aligned with the requirements I would think. And leaving that to some negotiation without those requirements seems like it just opens the door for bias. I want to hire engineers and keep them based on their qualifications, and negotiating abilities is usually not one of them.
The fact is, if there are reasons you're paying one candidate more than another, those reasons don't need to be secrets.
Surely if they are worth 50% more, you can give them a better title to go along with it.