The credit union, a private financial institution, has the right to determine which transactions they will allow
I don’t think they do. Just because I control a bank doesn’t mean I can tell anyone using the bank that they can’t purchase a subscription to a paper I don’t like nor can they pay for yard signs for a candidate I don’t like.
The idea that private banks can regulate the use of funds stored there is the steepest slippery slope I’ve ever heard of.
In a sane world this would be expressly prohibited by banking legislation. If you are a federally insured or regulated bank you would not be able to put any restrictions on the use of funds except for activities expressly illegal under federal legislation.