So I've been using the radio a bit recently and noticed that the android app will download all the songs I add to "liked from radio" regardless of "download over 2g/3g" being turned on. I believe this is one of the reasons the radio is burning MBs like crazy even when the quality is set to normal.
Going by the settings you'd think the app would not actually downlad the songs automatically when liking songs. If I say listen to 5 seconds of a song I already know and decide to like it in order to improve the station, I will end up downloading it as well.
I don't mind the songs getting added to a list, but I'd like to download the list when I'm back on Wi-FI and I believe this was the intention when you set "download over 2g/3g" to "off".