It's not Def Jams decision, they are paid in full by Google and from there on, it's Googles choice what they do with the albums.
Even though they don't count towards Billboard for the first four weeks, artists receive 70% per sale from Google Music, so at the end of the day you're putting more money in the artists pocket.
They REALLY need to make this service worldwide though, I'm in the UK and can't access ****.