Your bank most likely has an app on mobile. If you have Root and Xposed you can do crazy things to that app (and your phone). You don't use an app on a PC, you use their website.
It's not to stop you from abusing their systems but to stop scam victims from being screwed
One easy example is that you can get around the "no screenshots" lock many bank apps use with root, allowing you to potentially expose security vital information to people.
Should those of us who know what we're doing be allowed? Maybe.
But it's there to protect the old people who will run the .exe that's designed to root their phone and then let them hand over data that would otherwise be locked down so that doesn't happen just because someone called them and said they're from the bank.
One easy example is that you can get around the “no screenshots” lock many bank apps use with root, allowing you to potentially expose security vital information to people.
Nothing stops a scammer from telling someone to open their bank account, press prntscr on their keyboard, and paste it into their site. You don't see banks freaking out about that...
Most bank apps nowadays are just a webview wrapper over their web app. And they only have two reasons to maintain that app, to be able to make contactless payments with the phone, and to farm your contacts (supposedly for easier money transfers).