Just earlier in the film he was throwing tons of candy at those kids, essentially giving away half his store for free. Yet when Charlie comes in he makes him pay, seems kinda sh!tty of him.
Thats what i love about these high school girls, i get older they stay the same age.
Maybe we were seeing it as Charlie saw it. The kids with better off parents had free flowing candy and fun. The candy song was imagined by Charlie. I really doubt the kids parents would let them run a tab at a candy store nor would the shop keeper throw free candy around. The scene was showing the contrast of Charles life of working hard to bring home a loaf of bread.
That makes perfect sense to me. It goes with the theory that Wonka had a direct interest in at least that particular store. I can imagine kids would love to join a 'Candy Club' and get dibs on Wonka's latest creations.
The other thing of course is that Charlie could have just gone in there and got free lollies himself and did not know that he could have just gone in and joined them. The lollie shop owner might have charged the other kids other times.
All the other candy was tainted to make all those kids sick when they get home. charlie is a good kid so the candy shop owner made sure he only had the good Candy, which he had to sell.