It must be due to OS X. Or you can select a service you run on Murus Services, and define firewall rules in one click. Sebby says:

Murus Pro includes Vallum, an application-layer firewall. Price Free.

Configure and start the PF firewall in one click using built-in presets, use Murus graphical ruleset editor design tool or write fully customized rulesets using the advanced rule editor. So perhaps someone could mention specific things of interest to look for there too.