Even if API gave you strong durability guarantees, it still wouldn't mean much. Disk caches, big enterprise SAN attached storages etc, they can also "cheat", saying they flushed the cache while they actually didn't.
The API allows blaming the right cog in the machinery. Now everyone gets a Get Out Of Jail Free card because they can blame the kernel, filesystem drivers, userspace libraries, application developers, disk controllers, and whatnot, and thus nothing forces a single direction in that cyclic graph of blame.