"Free work" in this case doesn't necessarily mean "free work which is useful to the company" it just means doing similar things to what I'm payed to do normally for free with no benefit to myself.

I've gotten complaints from companies where I've done homework assignments for interviews that it didn't seem like I put enough effort in and my reply has always been that I get payed good money to do this stuff at work, they're getting the left over hours in the evening when I'm of course not performing at my peak. If they want work-level quality they should pay me for the assignment.