Missing modpoints again so quoting the AC +1 informative:
Speaking as a university researcher
I'm not disagreeing with the sentiment of your post, but in research ethics the concept of coercion is often taken much more broadly than it might be in typical parlance.
The idea is that if the incentives for research participation become too large, someone might not be able to rationally turn down an offer, and might be compelled to do something they do not want to do. I.e., you can coerce someone with rewards that are too large, just as you can coerce them with punishments that are too large. The idea is to prevent people from feeling like they sold their soul to the devil.
Where this gets complicated is that what is considered to be a coercive incentive depends on the potential participant's circumstances. So if you're homeless, you might feel compelled to do something you wouldn't otherwise do because you're desperate. I've been on research proposals where $35 or so USD was considered coercive because that amount of money was so large for the area of the world that they were recruiting from at the time.
I'm not sure how this intersects with this story--I agree that in itself, there's nothing wrong with recruiting homeless individuals. You also don't want to deny them opportunities that others have. But by the same token, you don't want to take advantage of their circumstances to make an undignified proposal something they can't refuse (not saying it is undignified, just that it probably needs more scrutiny, which it may or may not have had).