While Apple has made this work quickly and easily, it does take a little explaining to understand why it doesn't work between iOS & OS X.
Let me try re-wording it:
File transfer and communication/discovery between devices is not using the same connection.
AirDrop OS X or iOS 7.0.x both use WiFi Direct to transfer data between the two devices. WiFi direct requires no network connection, it works using the wifi of each device (not all hardware has this ability). How it knows where to send or receive that info. is directed by either Bluetooth or Bonjour:
AirDrop communication/discovery between devices uses:
iOS 7.0.x Bluetooth LE (4.0)
OS X Bonjour AD Hoc protocol
So, you see, they use a different way of communicating. While two OS X devices can use Bonjour to say "Hey I'm over here, here's my contact info., send it here", and two iOS devices can use Bluetooth LE to say "Hey I'm over here, here's my contact info., send it here", you can't take an iOS device using Bluetooth to OS X's Bonjour because they can't reach each other.
Does that help? (If I could draw a picture it'd be easier).
http://en.wikipedia.org/wiki/AirDrop
Why don't they just use Bluetooth? Because bluetooth is not meant for transferring large amounts of data quickly, it's too slow. It's meant to be a low power way of communicating between devices. Yeah, you can stream audio, but this is not the same as quickly getting photos to another device. Apple is making this quicker and easier than other people are doing it out there, and yet, maintain a secure, encrypted connection...that's why it isn't just over wifi or just over bluetooth.
For iOS, It works kind of like when you connect your phone to your car for bluetooth. The two devices discover each other (and then it's easier to just connect next time), but, here, instead of sending the data also over that bluetooth connection, it is using the much faster wifi connection.