The problem with any account-based music streaming service, from a corporate standpoint, is that end users are a shared password away from getting access to free media. Really, who hasn't shared their Netflix account once or twice? In an effort to prevent this kind of abuse, Google Music (likely at the request of the music labels) has instituted a cap on the number of devices you are allowed to deauthorize: Four. Per year. It gets worse, though.
For the uninitiated, here's how it works: you are allowed a maximum of ten devices that can be associated with a single Google Music account.