I've just completed an interview process (waiting for the decision) that was explicitly "this is a real problem we have", but also stating that I'll be paid for 8 hours work. I have no problem with that. It took about that long as well.
So I suppose if you're honest about it, and you give a fair estimate of the time required, then what's the problem?
I still disagree with that. are you being paid 8 hours at a pro-rated salaried rate? Well you're going to have to report that as income and take a 30% haircut off it. There's a reason contractors get 30% or more above the hourly-equivalent of a salaried employee.
So I suppose if you're honest about it, and you give a fair estimate of the time required, then what's the problem?