I was saying to Charlie that he should mention this in the post as someone would definitely bring it up.
Put simply: you're right, there's no need to have to encrypt the message serverside and it should be in the browser.

The only reason it isn't that way already is due to complexity of implementing it in javascript - but that's coming.

For now you can either trust us (and remember, it's only your posting key used) or simply not use the service until browserside crypto is in place.