It's a big difference between a device with a single partition on it and a device that has no partition at all. The approach you took can only work when there is no partition on the device, meaning that the raw device directly contains the filesystem with no partition table before it. If you have a master boot record with a partition table and then a single partition then only the partition will be made bootable. What is missing is a bootloader, in the same sense as you would need one when installing to a harddisk. Installing to the raw device directly only happens to work because the partition boot code that makes the partition bootable will then be written to the first sector so that the BIOS can turn over the control to it.
If indeed have an MBR and a single partition on that device, then this is not a bug. You're then just missing a bootloader.
