The question is, do they get a consistent stream of high-quality candidates via this process? If they do get enough to saturate the rest of their pipeline, why wouldn't they use a filter like this? I too balk at questions like this, but I think it's very reasonable to imagine it being effective enough for them.
Because wasting someone else's time isn't great, and we're all human beings and should probably care about that.
But more importantly for the faceless hiring entity itself, because it is very short sighted.
It leaves a bad impression at the very least to some people.
And more importantly, you are most probably filtering out a lot of people who want to avoid useless work (thus are more likely to optimize processes) and are too honest to simply copy and paste an answer off the internet (thus would likely be honest in their interactions as coworkers as well).
