For me, the line is clear. If the person you're trying to change knew what you knew, would they want to change? And so the placebo is ethical, because in fact, it makes people better when they believe. And the expensive wine is ethical, because it's a placebo, purchased by people who can afford it. But the fraudulent penny-stock scam is wrong, because the withheld information about the fraud being perpetrated is a selfish lie.
If you're okay saying to yourself and your family, "I tell selfish lies to the weak, the young and the uninformed for a living," then I guess we need better laws. I'm hopeful, though, that we'll figure out how to do work we're proud of first.