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.
The simple fact that they know it is a ponzi scheme is the gamble, amd there's ways of telling when and when not to invest by simply looking at the address on the blockchain, they can check the previous payout amounts and see the current balance so they can tell if they will make money or not, its a simple system.
For example you invest 1 btc and you will see our 1% fee come out instantly, and then payouts are every hour on the hour for 150 hours, if the pot runs dry then you lose if not you make 0.5 btc profit..