Internships aren't for making money, they're for gaining experience. Getting paid for them is nice, but the experience is the important part.

For some companies having to pay interns would just mean they wouldn't offer internships at all.

The reason interns are paid less (or not at all) is that there are lower expectations for interns and other staff usually has to take time to mentor them or check their work.

Maybe not the best example, but if a new paid employee says, "No, I've never used source control, what's that about?" it's a big problem. If an intern says it, you explain it and move on.