SANs are not free-form. A dNSName SAN is supposed to have an FQDN. An rfc822Name SAN is supposed to carry an email address. And, ok, sure, email addresses' mailbox part is basically free-form, but so what, you don't interpret that part unless you've accepted that certificate for that email address' domain part, and then you interpret the mailbox part the way a mail server would because you're probably the mail server. Yes, you can have directoryName SANs, but the whole point of SANs is that DNs suck because x.400/x.500 naming sucks so we want to use something that isn't that.