The only assumption that was made is that Galera cluster's SNAPSHOT ISOLATION actually implements SNAPSHOT ISOLATION. How is this not an assumption that should

The only assumption that was made is that Galera cluster's SNAPSHOT ISOLATION actually implements SNAPSHOT ISOLATION. How is this not an assumption that should be made by default? If the database user has specified a specific isolation level, then they have already selected a specific trade-off in performance vs consistency. Giving them a weaker isolation level in the name of performance is like giving someone a car when they asked for a train ticket.