Typically it comes down either to do with interest on the credit that you take out when you use your credit card, or it has something to do with the chance of their being a refund/chargeback if someone is trying to scam you by taking the Bitcoin and the money. Not that's something that is more common with paypal and less of an issue when it comes to most credit card companies. If you have made a profit then good for you, don't overuse it and get into serious debt.