Why would unions be exempt from the Affordable Care Act?

Here is a list of business and entities that are exempt from the ACA. I assume most of them fall under the 8% rule that exempts them

However, there are an inordinate number of unions listed and I don’t understand why. Unions are designed to protect employee rights, but it’s the business that the employee works for that pays the employee’s insurance, not the unions.

Can you ‘splain this to me please?