This has to do with data plans being incredibly lucrative, and finding more ways to encourage, or even force people to use more data, is cash in the bank. Limiting the storage ability at the hardware level, pretty much forces more bandwidth consumption if you have to use their network to access your data/files/content when you are on the go.
You may be on to something there. But there's also no doubt knowing what music you listen to and videos you watch is very lucrative to Google. I still say it has more to do with Google - the majority of people are on wifi most of the time which is why their data usage is so low.
I don't believe overages and higher tier plans are the money boon that people think - power users on this board don't reflect the normal consumer. Most users aren't going over or paying extra - they'll hit their caps, shut things down, get irritated and start shopping around.