There are laws against retaining user-data that the user has explicitly requested to be deleted and Google spends a lot of engineering time making sure, that is really the case (which is non-trivial e.g. with failing drives that you can't access anymore. You still have to delete the data from them, meaning you must know, at every point in time, the physical drive that a users data is saved on). I think 30 days is the guarantee they make.
