I assume that you could have asked how this was relevant to your position/future work, what they were evaluating with it, and if they could do something different that might not be so closely linked with the product.

However that said i have done work recently (10 hours) that was a task set out to solve an issue on the product - something that could 100% go into production but actually is very relevant to the position and skills i need to demonstrate. They are also of a size & expertise that makes me worry less about them just making me do it for un-paid work.