The application shouldn't be doing what it's doing, but it still is kind of hypocritical to complain that someone else is doing exactly what grub is doing. There is no reason grub couldn't make a little partition for its data (except for inconvenience). It wouldn't even have to know how to parse a partition to get to its data, it would just be there to mark the area on the disk as its own.
We've solved the problem of data contention on a disk a long time ago...
Why does that give it a free pass to write to places on the disk that are "reserved" or unallocated?
Grub may not operating within an OS sandbox, but it is required to play nicely with the OSes it is booting. The way to do that is to wrap your data in a partition or a file.
Well, you're right that grub-the-bootloader doesn't have a supervisor to stop it from writing to random places on the disk (since it's not running under an OS), but grub-the-bootloader doesn't actually write to those sectors, so I'm not sure what your point is.
grub-the-comman-line-utility writes to those sectors but it runs in linux userspace so it's really in the same position as the windows programs that also write to the reserved space...
We've solved the problem of data contention on a disk a long time ago...