I think it is considered illegal. I don't hate them, heck, I took part in some of those sites. but I will be really happy if these ponzi schemers get caught.
it would only be fair if Ponzi schemers get caught and you as well since "heck, I took part in some of those sites"
you were part of the ponzi since you knew what it was. and you helped them commit fraud, IF THAT was the case,
but since every single person that deposited btc in them knew it was a ponzi than it is not fraud. there was no promises of any return besides "if the next person send btc" do you see how technicality it is not a ponzi since every party involved knew what it was from the very start, there was no misleading information.