Come on. They didn't do anything wrong. They withdrew an offer of employment.
1. It could be performance related, and they just don't want to tell OP, 2. They realised quickly -- shock horror, things change quickly in startups -- that actually they don't need X guy they need Y guy. These things happen.
In situations like this you can fire well: give explicit reasons, make sure the person knows you're going to give them a great reference, consider paying them a notice period in excess of what you're obligated to. But really they didn't do anything wrong, and I don't think naming and shaming them is going to solve anything.