Earlier quoted context omitted.
Well, it is "trivial to set up" as per my link, and they don't offer it as an option, so apparently not. Perhaps they don't think enough people use PGP to make this a worthwhile option to add. But given the service they are offering, it seems like an obvious feature and quick win to me.
"trivial to set up" on your own mail server with you as the only user is slightly easier than rolling it out to thousands of customers, testing it, make sure backups are all working, write documentation for users, collect public keys of users, ... You can't just add a single line with your perl script to production and hope it works for everyone...
If that isn't true, then all features are non-trivial to add and you've built your systems and processes badly.