Taking money from dumb people should never be illegal. If you're dumb enough to believe that a Nigerian prince needs $50k to transfer $100m to the US. Then by all means, this should be legal.
Ponzi schemes should be evaluated on a case-by-case basis, but the burden of proof should be on the schemed.
For instance, if I ask you for $1k, and promise to return you $10k in 6 months with no proof or explanation of my scheme, that should be perfectly fine and legal.
On the other hand, if I have a porftolio with 100 other investors, and their phone numbers and you call them, and they say yeah this is legit, this man is a genius, blah blah blah. And I take you're money and flee the country, then yes that should be prosecuted to the fullest extent of the law.