The taxes thing was a pain when I worked in stores in Canada. You'd get about 10 customers a day presenting some kind of ID and claiming to be exempt from either gst or PST, and as a till monkey you'd have no knowledge or training in whether they were exempt or whether the IDs were legit. Add on people buying 6 cakes to avoid gst, and you might as well just press random tax buttons on the register

Probably that was the spirit of the law, but the letter of the law happens to cover the sons' situation. It's an unlikely edge case that the people who drafted the law could never have foreseen.