This is Monzo being a big grown up bank and going against their morals slightly in the interest of having a viable product in terms of generating a revenue for them.
If it defaulted to the zero interest option to help us all get the best interest rate of nothing then they wouldn’t make any money.
It is offering you 0% if you are actively and manually managing your account (minority of users) and you fully understand the options they are offering (smaller minority) and have an understanding of how interest rates work for you to know that it is better to select differently from the default (probably a tiny minority know all three), or if you aren’t aware of all of those things then you need to pay them some ignorance tax, unfortunately.