Here's what I'd say to that: "I enjoy doing work that adds value. If you have something valuable for me to do for your company, pay me what it is worth. If not, I'll be happy to sit in your air-conditioned office writing free software that has value for me. And then you will be able to see how much I enjoy doing that."

But you probably shouldn't say that out loud. You're not in a very strong negotiating position, after all.

This illustrates another reason why unpaid internship is pernicious: It doesn't teach anyone how well you add value. Because your time is unmetered and unpaid, you have no incentive to spend it wisely. And your employer certainly has no incentive to track it -- at least not out loud. (Imagine the conversation: "Wow, last week you worked 10 hours for free and saved the company $100k. Have a muffin!") What, exactly, is either of you learning? I guess your employer is learning how much value you can be coerced to add, for free, without you noticing or complaining. And you're learning how to look and act like a model employee.

Onward. "You can decide how much you want to work?" Yeah, I guess. I could also "decide" to live in a cardboard box and save on expenses. If I have a family, I could "decide" to never see them because I have to work two jobs instead. If I've got elderly parents I could "decide" not to take care of them.

But, more likely, an unpaid internship requirement will tend to select for young people with no family and a lot of time to spend at work. How convenient. Especially since overtly screening your employees for these traits is against the law.

Do you see why I'm tempted to call this "unethical" yet?

Anyway, none of this is to suggest that you're doing the wrong thing. We have to live with the hands we're dealt. Congratulations for finding a way forward, and good luck with your jobs.