For some public companies like banks, they may even offer ridiculous cash rewards to attract new customers, ignoring the fact that most of these new customers may not stay after receiving their rewards. However, there are business reasons to justify such business model:
- Even a small portion of stayed customer will, in the long run, more than compensate the cash rewards they've "given for nothing".
- Attracting many new customers will make one public company's short term revenue and balance sheet so much better, help them meet their seasonal or yearly goals, and make their management ready to receive bonuses

- Each new customer will bring some of their personal information to the company. Client data and privacies can really worth a lot.
Again,
before we blame smart customers to "abuse" a business promotion, we should not forget that it's probably the business mangement who is wiser.