They can still blame it on the safety of the driver. The box could be a honeypot for robbers.
The bus companies get away with this argument, so they are exempted from taking cash.
That is, they have to accept other means of payment than an app. For instance, in rural areas with 2-3 buses a day and a large elderly population that need the bus to get to the hospital (which is centralized to the city an hour or two away to save money), you can send in payment via bank (or maybe cash?) and get the ticket in the mail (which is only delivered every other day except weekends, again too save money).
So the focus of a cashless society in Norway is less about privacy and more about those who struggle with apps.