It is an issue of misrepresentation.
If the seller misrepresented in the course of making the sale, he should be held accountable for the losses, at least part if not all.
It's not just the difference between legality and morality.
We all know that some people have no morals, but always stay on the right side of the law.
The issue is how we can make such people accountable for their immorality and lack of ethics.
Low risk means
1) 5% chance of losing 10% capital, or
2) 0.00005% chance of losing 100% capital.
Unless the seller already had insider information, nobody will expected Lehman to collapse just like that.
Everything is in the prospectus.
Assuming you only received the propectus immediately after you signed up.
If you do not sue for misrepresentation within reasonable time upon receving the prospectus, the court will take it that you have accepted the deal willingly. Many Lehman investors were earning interests, and happy with the product for a period of time.