It's a security measure against the wrong crowd. If someone is going to use root for malicious purposes, they'll just take apart the app and remove the check for root. Blocking casual root users is doing nothing for your security.
If you were a casual root user, installed this app, and there was some back-door only accessible via rooted phones, it would be the bank's problem.
Whether this is a reasonable security measure or not, I imagine it was their line of thinking.
Casual root users are at enormous risk of malware, like running XP without a firewall levels of risk.