That's a FreeBSD "limitation". And it's only due to the way MBR partitioning works. FreeBSD uses Unix partitioning, which predates DOS partitioning, and DOS got it wrong.
DOS originally only supported 4 partitions (the 4 primary partitions in an MBR). It wasn't until later that the concept of an "extended" partitiong and "logical" partitions came about.
Meanwhile, BSD supported an unlimited number of partitions within a single slice from the very beginning. And, when it was ported over to the PC architecture, that "slice" took up a single primary partition entry in the MBR (but, it acted like an extended partition). Thus, why you can't install FreeBSD into a logical partition (then you'd have a FreeBSD partition inside FreeBSD slice inside a logical partition inside an extended partition and the BIOS would explode).
Since Debian/kFreeBSD uses the FreeBSD kernel, it uses the FreeBSD partitioning, and requires a primary partition entry.
Or, you can configure your disks to use GPT, which eliminates all that primary/extended/logical/slice nonsense.