Well I think it's a good idea to prevent web pages from just opening up a socket to some server, speaking whatever protocol they like. It could be easily abused to spam, etc.
Considering the move proposed in this thread to move even SMTP to HTTP, how would preventing connecting to SMTP prevent spam when one can still POST to SMTP-over-HTTP?
There is, however, https://www.w3.org/TR/tcp-udp-sockets/ - see section 10 for an example.