I'm not sure the information on this site is correct, but it seems kind of strange to have to pay twice when buying music. You're paying first for the actual mp3s and then you're paying a monthly fee when what you bought(and want to store in the cloud) exceeds 2GB?
Since the files are the same and there's no need for duplication on UbuntuOne's servers why would they need to charge you for storing them on their servers?
I can't be sure, but maybe they are giving everyone their own copy so stuff like accidentally removing things people already bought doesn't happen.
IANAL but it seems like this would make the data _yours_ instead of them providing it as a streaming service. Which means if they are e.g. forced by a court for whatever reason to not provide a specific song anymore (if the author plagiarised it for example), they can't take it away again from you because it's yours. If it were a simple service you could access to get a copy, they might be forced to not provide that anymore.
Since the files are the same and there's no need for duplication on UbuntuOne's servers why would they need to charge you for storing them on their servers?