The problem is that if they keep using them for politics or for other things that aren't system messages, people will start ignoring them. "Ugh, that's that bar that just shows dumb political messages."
Yes, system messages shouldn't be abused like this. They are supposed to be the exception not norm. Even though the message gets to the front of your attention on every single page, it would have been better if this was to be placed as a community ad or a blog post.
Seeing that this is directed to the US users of Stack Overflow, I am not sure what the rest of the world should be doing other than spreading the message.
To play devil's advocate, since SO has dumps of their data under CC, anyone from another country can still get the content.
The community and the system of asking and answering questions are threatened, the previous data will still be around.
