And this isn't a new feature they're adding. Remote streaming was already implemented and generally available to users.
I don't discount there being a cost in maintaining code over time, but it's not as though they have to spend any significant employee time on improving it. They already support UPnP and NAT-PMP to have the clients connect directly to the self-hosted servers.
It would be nice if they added NAT hole punching on top of that, but it's evidently good enough to work as-is in its current form. If they're not even running relays to support more tricky networks (which the linked support article has no mention of), keeping this feature free costs them literally nothing extra.