Maybe I'm just a slacker.
or your parents loved you enough and instilled you with an adequate sense of self-worth.
I suppose it matters if you are invested in your work. I've had weeks where I average 14 hours a day of work, but I feel like I'm learning, and accomplishing something. I've also had weeks where I barely work 40 hours a week, and feel like I'm still wasting my time.
Although, Susannah, I do feel like your "bosses" are taking advantage of the fact that you're quiting, and know your work ethic, and are making you pay for not being able to tell them to go screw themselves. At least it can only last through the week though, right?
I think the only thing you should worry about is, would these people give you a good reference, or intentionally give you a bad reference, out of spite?