I am not familiar with the term "proven authority", but I assume what you mean is that it is enough to prove the authenticity of the data in this case. Yes, a cryptographic signature of the pages content would be enough here, but then you would still have some kind of PKI and cryptography involved and in the end https is the best supported approach for that.
Defaulting to full https also has the advantage that you don't have to re-evaulate if you should be using https in the future, when you make some changes to the functionality or content of a website.
