Why is that? What security does it provide? This very project just demonstrated that you can send pretty much anything in HTTP(S) so any malware (I presume) you're trying to defeat can do the same.
Spam malware can scan IPs to find open SMTP ports to send spam so some ISPs and hosting providers lock down these ports to avoid getting their IPs blacklisted when a compromised host starts sending a bunch of spam through an open relay. I can see this being used to send email out from some restricted hosting providers.