If a potential employer demands to know how much I made at my last/current job, then in order to maintain a fair, symmetrical conversation, I would need to know

If a potential employer demands to know how much I made at my last/current job, then in order to maintain a fair, symmetrical conversation, I would need to know how much every one of their current engineers make. Anything less would be unfairly favoring the employer. Because they will likely never reveal such information, I think a ban on demanding pay history is completely justified. You should be paid what the company believes you are worth, not as little as they can trick you into accepting.