I didn't know OS/X used the decimal prefixes, but that just means it's less true, not untrue. There are still many more systems out there that use the binary prefix. I imagine most *nix, and not sure about Windows. And RAM is still power of 2.
I don't think it's terribly user hostile to express sizes as powers of two when you work with these kinds of numbers for a living, especially when it's near the bare metal (Erlang binary data type FTW!)
But I do think it's user hostile to have two different units depending on what you're looking at. If it were all decimal or all binary, it would be much easier.
Not true anymore. OS X (and I assume iOS) reports sizes in power-of-10 units.
If you think about it, it is really user-hostile to express file sizes as powers-of-two. Who can remember that a "GiB" is 1073741824 bytes?