Humans are not robots, we make mistakes and often do things that we later regret. I don't have the concept of morality worked out perfectly. However, it feels wrong to influence people in a way that increases the frequency of said regretful events, especially when potentially large sums of money are involved. It looks like over 200k went through the site before it shut down? Nothing to sneeze at.
You are right, there is a distinction to be made between those who created the site, and those throw money at it.
Some who throw money at the site are well off and are essentially using the scheme as entertainment. They likely would not be too upset if the money didn't come back. One could make the argument that these people are morally entangled to a degree, as they increase the volume of funds flowing through the site, which draws in additional people, and makes it more likely that someone who actually has financial or gambling problems is tempted to bet on the site. However, this is argument is somewhat weaker than the one against the actual creators of the scheme.