Doing something which is not prohibited, therefore assuming it's permitted, is one thing. But doing something which is against the law is entirely another.
The NSA have a long and consistent record of ignoring the law and just doing whatever they like. And again, these are only the things that have been publicly revealed. What is going on behind closed doors is almost certainly
much worse:
https://www.eff.org/deeplinks/2018/09/nsa-continues-blame-technology-breaking-lawhttps://www.washingtonpost.com/world/national-security/nsa-broke-privacy-rules-thousands-of-times-per-year-audit-finds/2013/08/15/3310e554-05ca-11e3-a07f-49ddc7417125_story.htmlhttps://www.vice.com/en_us/article/vb7eg9/the-nsas-12-year-struggle-to-follow-the-lawhttps://www.aclu.org/other/top-ten-myths-about-illegal-nsa-spying-programPerhaps now we have one more reason to avoid voice calls through phone and do that through Whatsapp or other VoIP services.
WhatsApp is owned by Facebook, one of the worst companies for respecting privacy in existence (probably second only to Google). Further, there has plenty of talk about Facebook building a "backdoor" in to WhatsApp to allow the government to decrypt communications when they like. Better to use an open source alternative like Signal or Element (which used to be called Riot).
In this case they have done that to the terrorists, but I am sure that there may be other incidents, when this was done to law abiding citizens.
They do it to everyone, all the time. Anything you text, email, say, write or post online, or search for, is being monitored and recorded. If you want to maintain your privacy, then you must take active steps to do so.