I believe it to be the same as any other gambling game, some people will win and some people will lose and its as simple as that!
Well not really.
When you are gambling you know when you can lose and when you can win.
For example you place a bet on roulette number 21, and other number is thrown - you see that you lost and you know why.
Ponzi scheme told you : you will get 150%!!! but at the end someone will not get their money and they don't know why - as they saw only 150%...
when you bet that germany will win World Cup and you see that they lost 2-1 you know that it was unlucly bet.
When you play Ponzi you never know when you are losing and when winning.
gambling and ponzi are not really the same.