Some cities in MO require emissions tests where they plug a computer into the OBD port to check for failures. Tuners have the ability to eliminate the CEL and emissions failures on cars with headers and no cats.
Yes, removing the cats and programming the ECU to defeat the emissions requirements for vehicles driven on public roads is illegal. I don't think it is illegal for offroad use and that is the disclaimer that most companies use to sell products.
There was a lawsuit in Florida over this:
https://www.thedrive.com/news/23833/...defeat-devices
I guess we'll see if this becomes a bigger problem nationally and how performance parts companies deal with the liability.