Bottom line is that it’s quite expensive for Monzo, but there are plenty of reasons people might want to use the allowance.
Particularly since debit card topups were stopped.
https://community.monzo.com/t/topping-up-own-account-via-monzo-me/46713
The policy is already quite restrictive, probably for this reason
https://community.monzo.com/t/monzo-me-payment-failed-because/51658
I don’t think Monzo are alone in keeping tight control of these limits. Starling doesn’t have settle up on joint accounts either.