Why does the professor mention the "free-rider" problem?

Narrator
Listen to a conversation between a student and a professor.
Professor
Hey Jane. You look like you’re in a hurry ...
Student
Yeah, things’re a little crazy.
Professor
Oh, yeah? What’s going on?
Student
Oh, it’s nothing . . . Well, since it’s your class ... I guess it’s OK .. . it’s, it’s just that I’m having trouble with my group project.
Professor
Ah, yes. Due next week. What’s your group doing again?
Student
It’s about United States Supreme Court decisions. We’re looking at the impact of recent cases on property rights, municipal land use cases, zoning disputes . ..
Professor
Right, OK ... And it’s not going well?
Student
Not really. I’m worried about the other two people in my group. They’re just sitting back, not really doing their fair share of the work, and waiting for an A. It’s kinda stressing me out, because we’re getting close to the deadline and I feel like I’m doing everything for this project...
Professor
Ah, the good ole "free-rider" problem.
Student
Free rider?
Professor
Oh, it’s just a term that describes this situation: when people in a group seek to get the benefits of being in the group without contributing to the work . . . Anyway, what exactly do you mean when you say they just sit back? I mean, they’ve been filing their weekly progress reports with me .. .
Student
Yes, but I feel like I’m doing 90 percent of the work. I hate to sound so negative here, but honestly, they’re taking credit for things they shouldn’t be taking credit for. Like last week in the library, we decided to split up the research into three parts, and then each of us was supposed to find sources in the library for our parts. I went off to the stacks and found some really good material for my part, but when I got back to our table they were just goofing off and talking. So I went and got material for their sections as well.
Professor
Hmm, you know you shouldn’t do that.
Student
I know, but I didn’t want to risk the project going down the drain.
Professor
I know Theresa and Kevin, I’ve had both of them in other courses ... so I’m familiar with their work, and their work habits.
Student
I know, me too, and that’s why this has really surprised me.
Professor
Do you ... does your group like your topic?
Student
Well, I think we’d all rather focus on cases that deal with personal liberties—questions about freedom of speech, things like that—but I chose property rights .. .
Professor
You chose the topic?
Student
Yeah, I thought it would be good for us, all of us, to try something new.
Professor
Maybe that’s part of the problem—maybe Theresa and Kevin aren’t that excited about the topic—and since you picked it ... Have you talked. . . to them at all about picking a different topic?
Student
But, we’ve already got all the sources. And it’s due next week. We don’t have time to start from scratch.
Professor
OK, well, I’ll let you go ’cause I know you’re so busy. But you might... consider talking to your group about your topic choice ...
Student
I’ll think about it. Gotta run. See you in class.