I had a chat with Neil Jenkins and Bron about this (two of the authors & WG chairs). That skew is intentional - their rationale is that there's way more email clients out there than email server implementations. So it makes sense to load more of the complexity into the servers.

Of course, until other email servers add support for jmap, the protocol has a bit of a bootstrapping problem. Getting support from gmail + microsoft exchange would be a huge win for jmap, and in the long run email as a whole.