This is absolutely true of HR departments that I have known. They would much rather have a stack of applications from possible employees they can interview - if & when needed, than to have nothing at all when a manager contacts them saying he needs an immediate hire.

I agree with you, this is unfair to a job seeker, hoping for work & thinking there is an actual job behind the posting, but it probably happens more than we realize. The remedy for the job seeker is to not get their hopes up & to apply to lots of jobs.