You just have to be within bluetooth distance; AirDrop doesn't require the same wifi network. It does require wifi to be enabled, because AirDrop creates its own wifi network side channel for the actual transferring of files.
Some (more and more) people simply have it disabled, because 4G is faster and more reliable than wifi. And if you don't have any bluetooth devices, it will save some batterylife.
This is very interesting to me. Does anyone know if there are material differences between Apple's and Android's implementations (in the battery savings sense)?
If you use the controls that slide down from the top of the screen (I'm at a loss as to what Apple calls it) and "disable" Wifi it simply disconnects from the associated AP and won't reassociate. It will turn itself back on at 7 AM tomorrow morning.
"Disabling" Bluetooth is similar, it will disconnect from paired devices, but BTLE is still available.
You have to use airplane mode, or go in to the Settings app to fully disable it. If that isn't discouragement, I don't know what is.
I have mine set to Contacts Only, and regularly use it to move photos between iPhone and iPad when on flights (and indeed other locations) without wifi or cellular.