can't any shmuck say they are a religion
Yes, they can. Personally I would not be in favor of classifying groups as cults in the United States. I don't think it's the government's job to protect people from "dangerous" religious beliefs. If you break a generally applicable law, you should be prosecuted, whether for religious reasons or not. But I don't want the government tellng me what I can believe or that I can't associate with other people who believe what I believe. In my opinion, that would be going back to the red scare era where people were dragged into inquisitions and asked if they had even been a member of the communist party, because the government didn't agree with the thinking of that group.
I don't want my government deciding what religions or philosophies are "weird" or even "harmful" (as long as not breaking neutral laws) and persecuting those groups. This country was bascially started by people who were essentially classified as a "cult" in England before the term became popular, so in my opinion choosing unpopular religious groups and making them illegal would be regressing 400 years.