Everything from dog fighting rings to child trafficking has dedicated non-profit organizations with paid or volunteer staff doing investigative work. They collect OSINT, do surveillance, and yes, pay informants. As long as you don't break the law in the process (like doing surveillance from private property), you can build evidence to turn over to law enforcement to help open a case.
Something important to note here is informants aren't always in the organization or participating in the activity. It could be a truck stop attendant who texts you when the same guy stops in to buy a case of water and then slips it under the roll up door on his cargo van, or a dishwasher at a cafe who noticed a group of guys with the same tattoo. Things that on their own don't rise to the level of someone calling the police. Payments help people who otherwise would keep their mouth shut because they are in financial situations where they can't afford to lose their job.
Why? It's not obvious why only cops can pay money for informants. You might say that they're paying criminals, but that's not clear either. The US has quite liberal free speech laws. You can say a lot of objectionable things without violating any laws.
I didn’t say they were paying criminals. I said they were paying KKK and Nazis. I made an analogy to firemen and arsonists to make the point that they were fraudulently creating demand for their services, not to suggest that who they were paying were criminals. It’s not illegal on the face to be in the KKK or to be a Nazi, though certainly member of those groups have committed crimes in the past. The SPLC was engaging in fraud (allegedly) by both supporting hate groups while raising money from their donors to counter hate groups. And they were setting up fraudulent bank accounts to hide the money flows.
