Then again, it might be hard to put people in a lottery where half of them are made to feed their own poop to their kids.
This looks like the first step towards larger studies.
Also remember that given this was quite experimental and could've easily had adverse health consequences, it had to be on a volunteering basis.
You've answered your own question there. As I understand it, pregnancy and childbirth related studies often have this issue as the moral stakes are higher and there are issues around consent. Also under normal conditions neither is actually a medical condition/procedure which needs treating and the benefits are often only seen over a long timeframe, making obtaining volunteers less straightforward. As long as everyone is honest, less than perfect science is still useful.
Suppose there's a hidden gene that causes people to be able to fly like superman if they're given their mother's poop. The gene also causes people to not volunteer in poop feeding studies, because the experimenters unfortunately placed the adverts on the ground. You find volunteers, you give half of them the poop and the other half the placebo. You find that none of them can fly, so you conclude poop doesn't cause flight.
If you make this probabilistic it gets less obvious to pick apart statistically, but same problem.
Which generally ends up being that lots of studies are about isolating the effect of one factor on young white educated men, but you can at least say you've isolated the factor as opposed to just comparing young white educated men to the general population