Two thoughts,
First what it looks to me like they are doing is making the field level for everyone. They will pay the same dollar amount, in this case what ever the dollar amount of 80% of single coverage is. If you elect more than single coverage you pay the extra. As a family member, you are not an employee. If the law in WI state they must pay 50% of the employee cost they are exceeding the requirement. Many employers pay nothing for added coverage whether it be family medical, or other choices you select.
Second, similar to michaeladams, my employer has a requirement if your spouse works and has a plan available to them, but in my case if the plan is offered they will not provide primary coverage to the spouse. In other words my spouse has to select her employers plan even if it costs more than putting her on my plan. I'm probably not stating it perfectly as it almost doesn't sound legal but the way it is written it is.
After bouncing a little from job to job and being self employed in between, I'm just thankful to have a job with someone that offers insurance.