For an employe, normally the job done is proportional to the wage...
But for an intern, it's different: the intern is not in the company to make production... he is supposed to be in this company to learn the job: so he is working mostly for himself, for his own knowledge/experience, not for the company, not for a wage (even minimum).
An intern costs money to the company: other employes have to take time to teach the intern, to watch his job, to clean his mess... So (I think that) a good deal is that for each hour of production the intern gives to the company, the company gives back to the intern one hour of training/teaching.
(of course when there is no waste of material and other collateral damages!!!)
So, the intern is wrong, but the manager is wrong too: if the intern is too stupid to understand that this internship is the way for him to really learn the job, even if it means working free, there is no need to pay him as an incentive, the only need is to kick him out!!!
Perhaps it will make him understand that the knowledge doesn't come by magic, but needs to work... sometimes hard.