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.