Nothing is as easy as it sounds. Even the most common thing undergoes a learning process of how it is done not to mention trading which requires someone's knowledge and skills before fully involved in it.
It would be weird for anyone who doesn't know much about trading would view it as a Ponzi scheme no matter how he or she perceives it ''to trade for some time and earn money". That doesn't make anyone calling trading a Ponzi scheme. What I normally hear from people is how risky trading can be and it is not something someone can enter without having the knowledge and skills.