If Apple want to sell their computers with the ability to only boot to MacOS for 'security reasons' then that is their prerogative, But that should be made clear in their literature if this is the case. If it is supposed to be able to boot to other OS and doesn't then it is not fit for purpose and needs either fixing or offer customers who bought it for this feature to send it back for a full refund.
Maybe Apple are thinking that only a small portion of buyers every run another OS on them so it is not worth supporting booting alternative.