I am of the opinion that businesses do this but i dont know why. Are they just trying to build a pile of resumes in case one of their employees starts screwing up and they want to replace him quickly or if they are just shopping around w/o being committed to hiring anyone or what?
Another reason may be to get customers in the door. You come in, fill out an application, then hang around a little to shop. You might buy more than you would normally, just to kiss a little pre-interview ass.
It might be used to gague the store’s popularity. If no one wants to work there, does the store have a bad reputation? Is it a bad reputation based on service or product? Does it attract “go-getters” or kids looking to make an easy few bucks?
Some businesses leave the “Now Hiring” sign up all the time because of their massive turn-over.
Trust me, you don’t want to work for one of those places. There’s always a reason employees don’t want to stick around.
This happened a few weeks ago. I arrive with a group of twelve or fifteen people to do an application and interview (this time of year, I’d EXPECT groups). The manager comes in, announces that they’re not actually hiring right now, but watch this video and fill out this application and he’s not interviewing today…we all went from a bit nervous to PISSED, but nobody said anything.
I haven’t heard back, and since I have NO previous experience, I was counting on being able to TALK to him, even for two minutes. An application from me looks hideous, but honest, given a day, I can learn to run a $*@^ cash register.
Heck, if they did call back, I’d probably tell them no thanks.
They do it because they are hoping they’ll find someone so desperate for a job and so, SO overqualified that they can hire them to take over 2 or 3 other peoples job and save money.
I have seen them, and it’s my opnion that it makes the perfect excuse for slow service. “Well, we’re short a few employees, but hopefully…” 
I haven’t done HR in that environment, but I suspect having people come in to request, fill out and submit job applications if you (or at least another nearby branch) are not hoping to hire, is a waste of time and bad for business. OTOH, telling people you are not actually hiring right now makes pretty good sense, in a Catbert way. You are not going to hire, or even interview, most applicants. Telling them you aren’t hiring right now means those you do call will be pleasantly surprised and those you don’t call will be less likely to hound you about why you aren’t hiring them. Overly persistent unqualified applicants are definitely to be avoided.
Is it a cruel attempt to remind your current employees that they can be replaced in a heartbeat?
When I was a theater manager in California we were informed by higher-ups to never have the sign up unless we were indeed hiring. I think it might’ve been an image thing but it might actually have been against the law. If you have it up all the time and not hire people, you leave yourself open to discrimination lawsuits, I think.