Take mobile data connections, for example: Most people don't want to pay for metered (by the byte) inbound traffic they didn't ask for that also drains their battery, but do want to be able to establish P2P connections for lower latency VoIP etc.
This is a firewall that's definitionally "not theirs", but that still also serves their interests, yet usually doesn't offer any user-accessible management interface.
So may I please traverse this firewall now, or is my use case still illegitimate?
If you are buying firewall as a service then request a user interface or change your service provider.
What provider would you suggest somebody wanting to make VoIP calls on their smartphone switch to that allows port forwarding of the kind you describe? And which popular VoIP app would support statically forwarded ports like that?
On the other hand, there is plenty of badly written networked software. I bet most of the networked software developers have no idea how to correctly plumb their software. They just open whatever connection, e.g. sockets, their OS provides and just run with it without care of the underlying layers. The OSI model theory in fact encourages this ignorance.