Sometimes I am not interested in knowing other people's political opinions. For example, back in the day, I liked many of my law school classmates until they started making known their political views (not their partisan views, but their conservative or liberal views) in torts and criminal law class.
Political bumper stickers are also uninteresting, although they help identify lunatics on the highway.
Should booksellers make their political views known? Sure, if they do so in a manner that will allow them to continue to make enough dough to keep the doors open. A bookstore can be more interesting if it presents an editorial position, similar to the way a newspaper will adopt a conservative or liberal position in crafting its opinions on the editorial page.