These sites must estimate the percentage of reversals they'll have.
Then add a premium price greater than that percentage.
Right. They do all the math and figure out the averages and what price they can sell the btc at and make profit. They just calculate the risk vs reward and deal with the chargebacks.
I wonder how big the market is for legitimate bitcoin buyers who purchase with their credit cards.
Probably big enough. The problem is there's no easy way distinguish them.