Earlier quoted context omitted.
JS crypto makes sense, but I'm confused as to why a browser extension is necessary—you can safely deliver the JS crypto solution over an https connection, so the critical component is keeping the user's private decryption key distinct from their login/password. Seems a shame to depend on a browser extension—you lose most of the advantages of webmail when you can't log into it from anywhere.
As tlrobinson says, the problem is that we dont want you to have to trust us. One of our goals is to minimize the amount of trust you have to place in us. Therefore, we want you to be safe if we are forced to try and steal your keys. See my other comment for more info: https://news.ycombinator.com/item?id=6245617
As you hint at in the other comment, there are various ways that third-parties could vouch for the code running on the decryption domain.