Borla said that any modification or replacement of cats, other to replace existing defective OEM cats with an OEM cat, is against federal regulations. Not State regulations, just Federal. Because of this their headers are only for racing purposes and they believe when it comes to racing there is no logical reason to have a cat.
I'm sure there are a million arguments to be made by different people on both sides and regardless we as consumers are more than willing to push the limits. I can understand even if it is against federal regulations why someone would want a set of headers and a lot of people would like to cut down on the sound and smell so that's probably why many manufacturers still offer them.
It's not illegal to sell or buy, it's illegal to replace or remove them for a car that is registered for use on National Hwys.