The interchange fee for credit card transactions paid by merchants should be the same 1.10%, regardless of which credit cards used by customers (Visa, MasterCard, American Express, and so on).
If I am not mistaken, the credit card terminals are by default linked to Visa and MasterCard network. If the merchants want to accept American Express credit cards, the merchants need to pay extra fee to their acquirer bank, to link the credit card terminals to American Express network (via Maybank).
If the merchants are willing to pay such extra fee, the credit card terminals from any acquirer bank (not necessarily Maybank) can accept American Express credit cards.
Correct me if I am wrong. As such, the acceptance of American Express credit cards among the merchants is not wide, probably due to extra fee to link the credit card terminals to American Express network, rather than the interchange fee.
For sure, some merchants may have misconception that it is more expensive to accept American Express credit cards, and that is not entirely true, because Bank Negara Malaysia stipulates that the interchange fee for Visa, MasterCard and American Express credit cards is same at 1.10%.
The interchange fee will drop to 0.48% on 1/1/2021.