They can be better than 128kbps, it depends on the quality you upload. But that's the artists choice and obviously if you want the best quality you should buy it. Also if you don't care about the quality but really like the song you should buy it :)
Do you know under what conditions the stream is higher than 128kbps? I've only done a small amount of digging around on their APIs and have not found anything but that.
