I am in no way doubting you, but this is such a common occurrence I hear about that I'd like to have a few links in my pocket to give people who either a) think this is a good idea, or b) have to experience this.
The other angle is understanding at which point an interview turns into an illegal full-day interview with actual work.
The point where an interview becomes illegal is when you do actual work. In an interview, they should run through mock scenarios to gauge if it will be a good fit. You do actual work after they hire you. Companies pay for training employees for a reason. Here's a flowchart for if you internship is illegal. http://www.moneysideoflife.com/illegal-internship-flowchart/ It's pretty much exactly the same in this scenario. Included in there, is a link to the Labor Department's regulations on the subject.
I am in no way doubting you, but this is such a common occurrence I hear about that I'd like to have a few links in my pocket to give people who either a) think this is a good idea, or b) have to experience this.
The other angle is understanding at which point an interview turns into an illegal full-day interview with actual work.