All created equal is fine. Advancing the interests of left-wing groups over others is not.
How about a pro-life book display? Or a pro second-amendment book display? Or a display of Trump bibles, complete with country music lyrics and a declaration of independence in each one? Since book displays don’t ‘promote’ anything, would you be comfortable with a library where a conservative librarian makes displays of conservative books but relegates the liberal ones to the back shelves?