I don't think this is inherently a problem. It's good for home routers to have sensible defaults. Blocking incoming IPv6 connections is such a thing. Opening a port in the firewall shows the same kind of intent as forwarding a port with NAT. The burden is on the router manufacturers to expose these options in a sensible way. My router for example has a similar UI to forwarding a port with IPv4 and opening the port for IPv6.

Using NAT as a firewall might work but it brings it's own problems. I find the IPv6 way better.