The scenario you give at the end is different. This isn't necessarily about finding advertised unpaid work - it's about getting your foot in the door and can involve offering to work highly discounted, or for free, in exchange for mentorship etc. Once you have the skills/connections from this initial step, you can advance your career. It's about taking calculated risks to advance your own interests - that otherwise probably won't get advanced as far/as quickly if you didn't take the risk(s). It's certainly not about working like a dog for someone trying to advance their own interests, while disregarding yours ("we need a website that's like Uber for X for $0")

But the big thing you're missing is the context - to get experience and exposure. If you're in a situation where it's difficult to get either, that's how you're getting compensated.

Again, I must restate, value comes in many forms. Money is simply one. In these situations you truly aren't working for 'free'.

The main thing is to put limits and goals on such activities. My ex got into an industry where unpaid interns were a common thing and it kickstarted her career. She did have to walk away briefly before they offered her a (low) paid position, and they she had to walk away once more before she got a FT position. This wasn't going to happen any other way, and she absolutely flourished.

The client I worked for didn't pay for my time but they paid for the time of my mentor. I specifically had 4 months @ 20 hrs. per week, they paid him 5 hours per week - this was detailed in the contract. I had no experience in Rails at the time and I sure as heck didn't want to fail at my first time at bat. It was worth it to me to take something like that on vs. a low rate without that assistance.

But yes, if you're just getting started in software as a college student and taking an internship, of course you should be compensated as fairly as possible, because that in common in that context - if you don't, you are being exploited. Totally different thing.