Are macros limited to the configured buttons only ?

I would like to setup a macro to power on TV and select HDMI4.  Pretty basic.  But the Macro edit will only allow me to do this if I already have a button configured for HDMI4.  Why can't I select any supported code.  Why do I have to assign a button for HDMI4 just so that I can select it for the Macro.  This is an unnecessary waste of a limited number of buttons.  Is my understanding correct ?  Is this a product limitation ?