It's possible that less companies would offer internships if they were required to pay interns. Interns are often just offered incentives, like the elusive "experience." Or even, in some instances, a potential to be hired on by the company. However, an intern is working a job without even being paid minimum wage, so that hardly seems fair or ethical. They should be paid, even if it's just at the minimum wage level.
Interns are basically working for free. This is in itself immoral. They are providing service to a company that this company would otherwise have to pay a normal employee for. They just use the intern label to save money on costs and circumvent the law. This is basically fraud. As the intern can't work another job at the same time due to conflicting schedules.
Interns basically enroll in internships to get an enriching experience on how the real world and work life , really is. Companies already spend precious time, money and resources for internships. If every company paid interns for their work, the companies would go bankrupt and the economy will be badly hit.
Interns enroll in internships to get an enriching experience on how the real world is like and companies are all ready spending a lot of time, money and resources. If all companies were to pay the interns, then companies would go bankrupt and the economy will be hit very badly.