As long as it's advertised openly, I don't see a big problem with it. It would probably be sold as a discount for shopping at slower times, though. It's a tried-and-true method of smoothing congestion.
Assuming a store with 9a-9p hours (every day), a 9-5 worker can shop 44 hours in a week, vs 40 they cannot. But that doesn't particularly line up with the busy hours. Around here, after 7 on weekdays and 5 on weekends tend to get pretty slow.