Why Credit Unions?
Spreading the ‘wealth’ (or should I say the grief)maybe?
My guess: Market share.
Credit unions have a very localized patronage. To not take IOUs is to lose out on the money those IOUs represent - a large fraction of their business. They don’t take the IOUs, they die.
The banks mentioned are big enough to refuse the IOUs with little ill effect. If anything, the IOUs are merely a pesky annoyance they’d rather not have to deal with, so it’s cheaper (and in the long run strategic) to refuse them.
Lots of public employees are members of credit unions.