Its how Google, Facebook, and nearly every other internet company makes money. Even companies like Amazon, who are in the business of selling you physical goods do it. For example, if you've ever searched for something on Amazon and then noticed that every ad you see for the next week is related to that search, that's why.

And ISPs have been doing it for awhile as well, the rule only went into effect a few months ago so what happened today essentially didn't change anything.

The NSA also doesn't spy on domestic US traffic. It has the ability to, but that's an inherent function of how the internet works.

The reason that surveliance is called a "wiretap" is because originally the police had to go out to your house, open up the telephone junction box for your street, and connect a physical wire to the phone line leading into your house which would allow them to listen in on your conversations. But that hasn't been the case for decades.

The way that modern telecommunications works - and has to work - is that every phone call you make or bit of data you send over the internet has to pass through your telecommunication company's router, and the router has to read or listen to every last bit of your communication, make a copy of that communication, and then send it on to another router that does the exact same thing. Essentially, every bit of data you send is already being read dozens of times by several different telecommunications companies, its just that there isn't a physical person monitoring those communications.

So when the police (or any other government agency) wants to wiretap you, its much easier for everyone involved if the company providing you with telecommunications services just has their router send a copy of the data you've sent to law enforcement as well as to your intended recipient. And the system that does that is largely automated, telecoms review a certain number of law enforcement wiretap requests to make sure there is a legitimate warrant but nowhere near all of them, so anyone in law enforcement with the authorization to place a wiretap could, in theory, wiretap anyone they want.

The ultimate check on that is that if an officer orders a wiretap but doesn't have a warrant, not only will they lose their job but nothing they collected from the wiretap can be used against you in court.

The NSA is also connected into that wiretap system, because there is a lot of foreign telecommunications that incidentally passes through the US, which allows the NSA to intercept them without needing a warrant (since you only need a warrant if one of the parties to the conversation is a US citizen or resident). And although its outside of the agency's scope, they could technically act like a normal law enforcement agency and obtain a warrant to wiretap a domestic communication like any other law enforcement agency.