Comment 4 for bug 381352

That first partition *is* the partition table (aka partition map), that's why you can't remove it. "The Apple partition map is unusual in that it defines itself as one of the partitions on the disk."

If you delete it, you get "Partition map has no partition map entry!", which means the partition map lacks an entry referring to itself as the first partition (because you deleted it).

I just toggled the bootable flag of it using the command-line parted tool, and now it keeps saying "Partition map has no partition map entry!" and not letting me toggle it back :/