Companies always say they give experience during an internship. They say this experience will help the interns find a good job later in their lives, an advantage they wouldn't have otherwise. Therefore the company feels that this enhanced position in the job market should be the reward for internship, and money should not.
But this is not the full picture.
It is in the interest of the company to have experienced workforce. They benefit from the interns' growing experience by creating a job market where they can choose from experienced workers. The company gains advantage when the job market has many experienced people to choose from. The internship benefits the company as well as the intern – beyond the direct benefits of the work done, which only benefits the company.
So the experience given to interns benefits both parties – the entire industry, in fact. Therefore this experience is not a valid reward for the work performed. The work has to be rewarded separately. With money.
But there is another opinion business leaders often have: interns are usually too young and inexperienced to be considered a serious workforce.
Alright, then hire someone who isn't too young and inexperienced, pay them a proper salary, and stop abusing young people just because they are young.