I think you're confusing data compression with audio compression, as treewind pointed out in his post above.

iTunes sells MP3s, which are an information-compressed audio format.

That's a whole different kettle of fish to broadcast radio using audio compressers, which normalize the volume level of the music they play, reducing the dynamic range, and making everything overall louder.

It's a bit like talking about the volume of information present in something as opposed to the volume level of the audio of something.

Personally, I find it quite hard to spot the difference in terms of high-fidelity between an MP3 recording, a WAV recording, an AIFF recording and a CD recording. You have a pretty sophisticated pair of ears if you can tell that the Radio 3 broadcast you're listening to in your car is a 16-bit MP3 of a Tchaikovsky concerto rather than a compact disc of a Tchaikovsky concerto.

Whereas you can spot the difference between an audio recording that has used a compressor on heavy settings and one without right away.