The candidate does not get paid to solve the interview test, but it could be seen as a collaborative effort on both the company and the candidate, since both put time and engineering resource in it.
If you do hire them then I guess it becomes fine, but otherwise definitely immoral.
The problem is that we’re fleshy meatbags and easily influenced. It’s possible we internalise answers to interview questions and then “spontaneously” come up with them later, without _knowingly_ stealing the idea.
This happens sometimes in comedy, and the comedian apologises. You can usually tell the difference in comedy between plagiarism and accidental theft because the joke is delivered much differently but has the same core. It would be harder with interview answers.
Or, put differently: if I’m the candidate and they end up “stealing” my solution, that’s a small price to pay to avoid working in what is likely a terrible environment.
Not that anything will stop companies from doing this, and not that I think it ever really happens, but for it to be morally OK I think you’d have to both get their permission and compensate them fairly.
Before I work for you, my ideas are mine even if I write them on your whiteboard.
Why would I trust the answer more than the person?
Why wouldn't you? The truth of statements is not a function of the people making them (though of course that can often be a strong indicator of truth).
I've had candidates use useful Python libraries that I didn't know about in tech tests before, but that's pretty much the extent to which I've learned anything helpful when recruiting
However some whiteboard scribbles are not the same as working code, so it is likely easy to make an implementation that does not infringe in most cases. Not that it really matters because it's highly unlikely that anyone is going to sue over it.
To ask interviewees a question about an actual issue you have and see what solutions they come up with is clever. You get more input for free and you may find someone you want to hire.
I don't see a problem as long as you are really looking to hire someone and not advertising fake job openings.
If it can be regarded as an intellectual property, does that mean that the company would be infringing on the candidate’s right?