Most likely it is simply the case of the clerk/teller just not knowing what to do. They have procedures they have to follow and they could be fired for not following. If it isn’t specifically documented what to do in this case, they probably played it safe.
from the article:
He says a bank manager told him he could either come back to the bank with his wife or open an account himself.
The bank manager is the moron who turned him away. He should have either allowed a toe print or called his boss. As it is, he could be facing an ADA lawsuit. That is NOT playing it safe.