Comment 23 for bug 1059827

Actually, Phillip, BtrFS requires 2MB for a Grub2 installation. So, while you are correct that it does start at 1MB, the partitioner does have an error in not including that the rules for BtrFS are different. The normal 1MB is a part of the normal MBR rules. Since Grub2 requires some extra installation space for the BtrFS partition information, GParted should warn when BtrFS is used and a partition is started less than 2MB from the beginning of the drive. For everything else, 1MB works fine.
So, this would be a problem with the partitioner not realizing the rules are different and the installer not seeing that the partitioner made a mistake and that Grub2 can not be properly installed inside of the 1MB space for a BtrFS partition.

Ext4 and other partitions that are not BtrFS have no problem with the 1MB start point.