Because in six months the league is going to be best served by having Brady on the field.
Now, that said, this MMQB news is disappointing because Peter King and co. are basically open-faced shills for the league, and if Goodell and company think that they have to fine/suspend Brady in order to keep up appearances then it's hard to argue with, since the offense pretty obviously doesn't warrant any sort of suspension (and certainly not twice as long as Ray Rice's initial assault suspension).