This isn't a law that's strictly enforced. There's not enough sapients with knowledge of that law or the ability to carry it out. So it's a law that gets broken quite a bit. Izzy's not the only sapient traveling with stolen gedutu technology. Some of them have even gotten around that pesky selection process.
Cecile can sometimes be a bit of a stickler. Not that she's run into anyone who's done this before but she's at least heard about it and knew that if she ever caught it happening she wouldn't let it slide.