The thing is, people do tend not to abuse stuff like this. For instance, where I work Coke, Diet Coke and lemonade are free refills. But people don't buy one drink between 4 and share it. Not most of the time anyway, I mean, yes, there have been a couple of occasions where it has happened and I do have to point out that the rule is one per customer, so the others can either order another drink for themselves or go without. But generally speaking, people tend not to be cheap like that.
I guess what I'm saying is just because there is a system there to be abused doesn't mean the majority will and cause it to fail.