I got a bachelors in Computer Science and then worked for 15 years before going back and getting a masters. While there I was asked to work for free on numerous occasions, we can call if different things, but that's how I saw it. Already knowing the value of my labour I finished my masters and then left. I definitely had the option to continue with a PhD, and I'm almost certain that had I applied I would have been accepted. But I just wasn't interested in being exploited.
I remember one class which I dropped in which the entire class was project based, and we were to work on projects that aligned with the research of the professor. On the first day there was no syllabus given, so I asked what the readings were and received a grumpy response basically saying, "Readings, yeah there will be some readings." Or something similar. It was incredibly clear the instructor just wanted grad slaves to advance their stuff. I dropped that class, but I could cite numerous other examples.
I paid for the privilege of working for free and learned some stuff along the way. Maybe I should have spent the two years and money on travelling and seeing the world instead. I likely would have learned more, but I wouldn't have this fancy grad degree.