To get all your existing music into Google Music, you have to upload it. I think this is what OP meant in a roundabout way. Uploading isn't the same as syncing, and it's a one time thing, not a continual thing. It's Google's shot at Apple for requiring you to sync you devices to a computer, but iOS 5 has kind of taken care of that with wireless sync.
It could also be a poke at iCloud, as it is my understanding that it isn't true streaming in that it caches stuff on your device. I don't know how Google Music handles this.