Because it’s their software? It is well within your bank’s rights to deny you access to their online banking system for pretty much any (technical) reason they choose; why are you entitled to run their app on what they deem to be an insecure platform? If you don’t like it, either pick a different bank or deal with not having access to their software.
Freedom cuts both ways here; if you want absolute freedom to do whatever you want with your device, why should software vendors not have absolute freedom to choose what platforms their software is permitted to run on?
> why are you entitled to run their app on what they deem to be an insecure platform?
For starters, because they're wrong, and they're wrong in a way that makes their users less secure. Allowing the use of Windows or Android with open CVEs while blocking completely up-to-date Linux or aftermarket Android ROMs clearly shows that this nonsense is contrary to security.
> If you don’t like it, either pick a different bank or deal with not having access to their software.
And that's the next biggest reason: Customers don't have the same amount of power that the companies have, so it's perfectly reasonable to tilt things in the customer's favor.
They do have that right as long as they do not sabotage the integrity of the system. Personally I would quickly use other software developers and banks. Competition would sort that out quickly.
Today banks don't want to be liable for user error that lead to scams. They adhere to such policies out of fear, but they don't help as the most prominent victims of scams are still the same people, open system or not.
Freedom cuts both ways here; if you want absolute freedom to do whatever you want with your device, why should software vendors not have absolute freedom to choose what platforms their software is permitted to run on?
Obviously none of this applies to FOSS.