Avoiding patrons from being tracked/potentially in the future prosecuted for their reading is professional ethics for librarians, pretty much regardless of political alignment.
I'd appreciate it if you could name the online system your county uses. (Or if you can provide a link.)
I'm not sure what backend they use for data storage, etc.
The biggest professional organization for them in the USA (the ALA) is not officially politically aligned, but some of their core positions, notably privacy, intellectual freedom and anti-censorship have of late have come to be seen as politically left and in the States especially more aligned with the Democratic party.
Demographically librarians in the United States do lean liberal but I don't know that they do at any higher rates than other educated people.
Finally, this is not just an American thing. The IFLA has very similar positions on privacy, intellectual freedom and censorship and monitors those positions around their world.
There was a case a while back where a woman poisoned her husband. The police found her thumbprint on a book on poisons in the library. I don't think they used library records, they just checked the book itself.
I also recall that there was considerable pushback from the librarians when the government demanded they hand over all borrower records.
Next time I go to the library I'll ask what the current state of this is. I hope their practice is to purge records when you return a book.