Buying it is the exact reason they can afford not requesting a warrant.
The logic chain is as follows:
You wilfully pay for services and content with your data (you don’t pay with cash, and you keep using services that you know use your data instead)
Those services do use the data as advertised (for advertisements, too)
Anyone can then buy the data. Your data. It is by design, and you agreed to it.
The NSA is among “anyone”. They absolutely have the right to buy the data that you sold for services. The irony in their case, is that they are using your own money, that they got though taxes, for that.
They would not have the right to get that data otherwise. And for the most part, they probably wouldn’t get it (the amount of data generated by the surveillance capitalism is properly staggering and mind-blowing). But you sold it, and it’s fair game, so they might as well buy it with a tiny fraction of your taxes, right?