> But I've never even heard of retail bitcoin fraud with zero-confirmation transactions.

Then you are simply not very well informed. Problems with BTC transactions have happened repeatedly. Here is a great example of a successful 10k double spend during a chain fork: https://news.ycombinator.com/item?id=5372956 It was was made possible buy slightly different behaviours in the different client versions. The core developers had to get on IRC with the miner using the 0.8 version and talk them into giving up their (correct) chain to resolve this problem. Currency of the future!