On the one hand, that may be an artificial limitation. If this turns out to have some bug that overwrites the encryption keys, they could of course say "we warned you", but it would not be good PR. Also, if beta developers report intermittent smaller data-losing bugs, they might want to study the affected drives to see what went wrong with it. Not having encryption enabled on them will make that a tiny bit easier.
On the other hand, if the new ability to partition drives with flexible partition sizes includes separate encryption keys per partition, and encryption/decryption is done by the block driver, they may have work to do to keep that block driver informed about what blocks should get encrypted with what key.
On the other hand, if the new ability to partition drives with flexible partition sizes includes separate encryption keys per partition, and encryption/decryption is done by the block driver, they may have work to do to keep that block driver informed about what blocks should get encrypted with what key.