Boot loaders belong on the MBR or master boot record for a drive.
There are no MBR’s on partitions; if you write to a partition you’ll be overwriting data on the partition (the first sector of the partition).
The first sector of a physical disk by convention (back to early 1980s) is reserved for the MASTER BOOT RECORD, ie. it’s related to the drive itself. It’s what PCs use to boot.
The first sector of a partition will not be the first sector of the drive, but a subsequent sector intended for data (be it user data, or file-system or other meta-data).
It’s preventing data-loss by not allowing you to write to a partition (as that could occur - it’s invalid); and it’s only a sign of better validation of user-choices (preventing users from making wrong/bad and potentially destructive choices).