Same thing when a college coach gets blamed for not keeping an eye on his players. They're young college kids. Some of them are going to party and get in trouble. It's literally impossible to watch everyone all the time.
Maybe I'd change my tune if teams started forfeiting draft picks. However, as it stands now losing 50% of the salary a player forfeits seems like a non-issue. Some player gets suspended a few games for smoking dope and the league takes that 50% salary and donates it to a drug rehabilitation rather than stuffing it back into the the team's pocket. I'm all for it. (I don't know for sure this is where the money will go but this is the norm for NFL fines.) Make something good out of something bad and it's not like it has cap ramifications or would affect competitive balance.