There are various reasons a provider may not be totally in favour of encouraging end-to-end encryption. This service being a perfect example. Spam filtering and phishing detection, which I suspect is more important to most users than encryption, is rendered almost impossible anywhere but the end-points by such encryption. And, whilst the client may be able to have a reasonable stab at performing some of this filtering, potentially assisted by certificates on encrypted mail (though even this is questionable for unknown senders), in reality it will likely always be inferior to the service a large provider could give with dedicated teams of analysts and the ability to detect trends across millions of different users. Fundamentally, if PGP became universally and seamlessly available, it would not take long before phishing and spam started abusing it to evade current filtering. Not saying people shouldn’t use such encryption for sensitive mail, but for most users, especially those prone to mindlessly clicking links / opening attachments from unknown sources, I simply cannot see the pros outweighing the cons.