They cannot cheat everybody, all of the time. They can only deceive some of the people, some of the time. And that is sufficient for staying in business.
This reminds me of a story:
The Hodja had seven monkeys. One morning he told them:
"I will give each of you three figs in the morning and four in the evening. That will feed you for the day and there is no more."
The angry monkeys started to scream.
"All right", he said, "I will give you four figs in the morning and three in the evening."
With this, the monkeys were appeased.