IMO it makes it less work for the manufacturer to leave them that way for biasing. There's nothing holding them back from leaving the sockets internally and having the biasing done at the top or back of the case. It's just a matter of wiring it to a different location.
The only reason I brought it up is I've seen DIY amps that have easy access to the inside of the case where you can bias the tubes. But the best looking amps I've seen have a spot where you can bias the tubes from the top or (my preference) the back of the case.
Now if the 6c33c socket prevents it from doing that then I understand why they left them outside. They look really good on the Zana Deux, but I don't know if that's an auto biasing circuit or not.
edit: nope looked at the pictures, they could have easily wired the biasing posts to a different part of the amp.