To be specific about it, iOS has APIs that notify the application (rather than the user) about the device's network state, whether its cellular/wifi/absent. Application developers are expected to control app behavior given this information, but only some subset do.
This is still not quite as fine-grained: Android allows you to specify that some WiFi connections should be treated as 'expensive' in the same way that 3G connections are expensive, so even if you are tethering over WiFi you can still limit your data use. My S3 also has (I'm not certain it's stock) functionality to limit applications' background data use to WiFi only. I've turned that on for a couple of data-hogs.