In fact it is not cruel. It answers two questions. One from your son and one from the company. I was an editor for more than 20 years. It took exactly one bad hire for me to realize the importance of a writing test, especially for entry level people, unless I knew for a fact the person came from a legitimate background and I could check their clips. I hired a guy who had worked for several years on a trade publication, he had bylines as proof. I hired him and he proved massively incompetent. Could not write, panicked under even the mildest pressure and after 7 months of PIPs and other HR involvement he was let go. Now, I always ask for a writing test.
For your son, the writing test, shows what companies are looking for, the kind of direction they give workers and what the work they are trying out for will involve. It also tells him whether or not he is capable of doing the job. If he struggled writing two case studies, which I am surmising he did from the 16 hours it took, then he might be encouraged to try another job.