There is a cost of the money and it isn’t 0.3% for two months. Banks have to get that money from somewhere and it costs money.
However, that is not the world we live in. Most people carry balances a majority of the time, with interest rates significantly above "the cost of money".
Further, the banks (really: card issuers) (in the US, at least), are collecting 0.5-4% of all transactions as fees.