I'm writing this from memory, so some details may be wrong but: most high end ssds have dram caches on board, with a capacitor that maintains enough charge to flush the cache to flash in case of power failure. This operates below the system page cache that is standard for all disks and oses.

Apple doesn't do this, and use their tight integration to perform a similar function using system memory. So there is some technical justification, I think. They are 100% price gougers though.