If we don't stop this now, and don't make it explicitly illegal, and actually punish the people involved (otherwise they's no downside to continuing it), then it will get a lot worse in the future.
Also, how can US law be so asymmetrical? It's illegal to look into someone's mail, which just includes one single conversation, but it's not to look at someone's real-time conversations for a long time, which can include all sorts of topics, and then multiply that by millions of people.
If looking into just one mail conversation is illegal, how come mass surveillance isn't immediately seen as massively illegal?