Payroll taxes scale down as the employee's pay rate scales down. This was a flat amount per employee-hour.

Exactly, the Seattle tax punishes low-margin employers of blue collar workers much harder than it hits employers of high-salary white collar workers. It's as if it were a carbon tax on labor designed to speed to adoption of automation.

I wouldn't call it paid for by employers. It's a tax on your employer to employ you. It goes directly into the cost of your employment along with your salary and your benefits. If it weren't for this tax you could make more money. You are paying it just as much as you are paying "your half" of social security.