At first I was in love, but price points don't make any sense - 130$ difference between 64GB eMMC and 256GB SSD is a price of a 1TB SSD for the consumer - not HW bought in bulk when manufacturing a device.
I'd be the first one to buy 399$ model if I was able to put my own SSD inside.
Oh, that's simple. They want the cheap option so they can advertise how affordable it is ($400 starting! What a great deal) but they don't actually want to sell it for that price so they instead cripple the base model and hope people will go for the more expensive variants.