It’s not against the law to sell cow brains at all, just hard to find them in the regular grocery stores. Try ethnic food markets, I get mine in California from a Mexican Carniceria, (butcher shop). You can also find them in many Asian or some halal markets. The danger from spongiform encephalitis is minuscule in America, you’re much more likely to get food born bacteria infections from ground beef like E. coli, listeria or salmonella. These occur thousands of times per year and have a death toll attached. Mad cow disease is more of an issue in Canada and the UK.
There have only been six cases of spongiform encephalitis in humans in the US according to the CDC’s records. I don’t think the infections all originated in the US. And if I remember correctly only three infected cows in the US have ever been found. It can occur randomly although it’s pretty rare. I read one of those three cows was a random occurrence and the other two were somehow infected but I can’t remember how.