This is extremely concerning. People's hardware should not be remotely edited to broadcast political slogans, even if this particular slogan is a good one.
I don't think that really mitigates the concern. If my microwave started saying "support our troops!", I wouldn't be comforted by the fact that I can throw it out and get a different one.
But your microwave doesn't connect to an online service that you don't pay for. Nobody is forcing you to use Siri, or any of Apple's online services.
It's more similar to a website you use saying "support our troops."
If you don't like using an always-online constantly updated service, then you have the option to not use it. You pay for the convenience of Siri's knowledge and functionality by giving up any control over how the system responds and acts. You can always turn your internet connection off when you use Siri, and thus only use it for things like playing and pausing music.
Well, it doesn't yet. I suppose my smart refrigerator would have been a better example. As more and more components of our lives get a screen and Wifi connection, we're going to need better norms than "online services can do whatever they want".
That is not at all what is happening here. They are not 'remotely editing' your hardware any more than a corporate website who puts a support banner on their website for some cause. Siri, much like websites, is a service. Like a website, you are free to not use it if you so choose. Nothing has changed on your device, and if anything this is less of a feature than a banner on a website because you have to specifically invoke these actions.
I see Siri as an input feature for my phone, not a remote service. For Siri to pick and choose what it wants to accept feels like my keyboard refusing to type out phrases it doesn't like, or my monitor attaching little popups to content it finds offensive.