That a BS rumor.
It's a $10,000 fine if you are business and you are caught red handed as it only the maximum fine not the fine in every case. For general public it's more like $500 and once again you would have to be caught with the hammer and chisel in your hand. Cats only last around 7 years or 70,000 km's (new ones might last longer) and then they generally get spat out the back of the pipe after rattling around in the can for months so it would be very hard for them to prove that you did it. Also if you are running rich then they don't last anywhere near that time as the extra fuel burns them out, i don't know how many cats have died on dynos before the tuners get the fueling right, 5 really rich runs and you cat would starting to become a molten mess.