It feels cynical to see this as a punishment when it's such a specific use case that does demonstrate deep integration with Siri. Maps, Messages, etc. use Siri for their interactions.

I am sure that there was a meeting where they decided what to do when Siri was off and somebody decided (very possibly with ulterior motives) not to split the feature set - all or nothing. However I don't think the challenge they were faced with in this hypothetical meeting was an easy one.

The alternative is you open the Messages app and you can't send messages. You open Maps and you can't get directions (unless parked). Sure, I get that they could show a screen saying "Sending messages is not available when Siri is disabled" but now you're hitting error messages while driving.

Anyways, the main reason people would disable Siri is accidental activation, and Apple provides all the toggles needed to avoid that without disabling the core components needed for CarPlay.