The analogy is if you want to belong to a society, or an organization in which you will be given benefits, very often you are required to join, pay dues, or otherwise commit yourself, and not just be a tourist who soaks up the blessings without giving anything back. It still stands in your right to work states, as even there you would have to bow to numerous dictates, such as electrical codes, zoning regulations, business license requirements, insurance rules, etc, and also pay taxes and assorted fees. You might not like the politics of that particular state, but you would have to go along with it, if it was a legally elected government. The comparison with unions is direct, it's just that you are horrified you might end up in some "leftist" organization. If you don't like that though, you can attempt to change it- vote in the union elections.
I'm a little curious now as to your African outlier you have Googled up, just for interests sake. Not that it is going to change history, or the reality of unions today in places like Europe or Australia, the very places that come at the top of the heap in quality of life surveys, over and over.