> Why would you need a tax? When products are originally designed with an awareness of how people with different abilities will use them there is no extra cost involved.

That is, unfortunately, completely untrue. While there are some decisions that can be made (to improve accessibility) that do not impact the cost of development, there are many that do.

While it's true that each small change tends to have only a minor cost, those costs add up when you realize there are thousands of them. They add development time, testing time, and complexity (which impacts maintainability and associated costs).

I think it's worth it in most cases, but it's certainly not free.