The trust can be more automated in this case than it can with CAs. The difference is that when a CA does something it shouldn't, like generates an extra cert for a government to use in spoofing a site, nobody knows about it, unless they mess up. Double spends on the network can be monitored and stored for history. Merchants can and will share information on instant provider trust with eachother, so they will essentially be able to build up a credit history on a given instant provider without really knowing who they are.