Very strange, but also doesn't seem like a deal breaker for me. I would be interested to know why they did it assuming it's not a typo as some in the forum suggest.
Fairphone
Fairphone is a Dutch electronics manufacturer that designs and produces smartphones. It aims to lower environmental footprint through minimizing the use of conflict minerals in its devices, maintaining fair labor conditions for its workforce and suppliers, and allowing users to maintain their own devices.
Some people who were at the launch event said in the forum that they asked about it and were told that it's a cost thing. Although, some other people say that the cost saving is just a few dollars tops so there might have been other reasons as well.
tinfoil hat on
They did it to be able to sell a cheap USB 3 module later down the line to demonstrate how awesome the modular approach is.
I read on the forum that they said it's unlikely a module upgrade would be possible, unfortunately