I don't know why this topic is difficult to understand. Even if your credit card doesn't carry an annual fee, it will usually include a foreign transaction fee if you're converting from a foreign currency. That is, you are charged a percentage of the purchase price when you make a foreign transaction. There might also be a foreign exchange fee. You won't get the interbank rate at the time of purchase. It will be something higher, often 1 percent, Then there are ATM fees, cash advance fees, late payment fees, etc.
Merchants are also charged a fee, usually 2.5 percet,
Not all companies charge all of these fees. The OP wasn't talking about an annual fee but a foreign transaction fee. That fee is built into the purchase price. Again, not all cards have foreign transaction fees but they are common. It's also common to charge 1 percent above the prevailing exchange rate. You may be waived these fees if you are a priority customer.
It's similar to the fees charged when withdrawing cash with a foreign debit card. Then you pay the bank fee, which I believe is currently 220 baht, and the foreign bank or CC company might charge a foreign transaction fee. The Thai banks try to induce you to also use a "dynamic conversion," which would result in another fee of 5 percent.
Only with cash are there truly no fees, which is why banks now want to start charging for the use of cash.