When you choose to use this form of payment, the merchant is able to pass the rewards to you. Retailers operate on low margins and a high cost of their operation is the credit/debit card merchant fees. When you use a credit/debit card at the station, the merchant pays about 2% of the amount you spend to the credit/debit card company. That is expensive for the retailer.
Since most gas stations are not equipped with the technology to accept a check, and consumers are not carrying checks when they go to the station, payment by check is not practical. However, in essence, the ACH payment account is an electronic check which you authorize to electronically debit your bank account.
The retailers would rather reward you directly than pay the higher cost to the credit/debit card companies. It is a WIN/WIN for both you and the retailer at whose business the transaction and the exchange of goods takes place.