As the author of You Can't Win If You Don't Enter (Canadian Edition - I am almost finished the American Edition) I can answer almost any question regarding sweepstakes and contests in both countries.

The reason we have to answer a skill testing question in Canada is sweepstakes are illegal. Contests and lotteries are not. To get around the law, companies running a sweepstakes ask the potential winner a skill testing question, there by legal definition making it a contest, and upon giving the right answer, they can declare the person the winner of the 'contest'.

Also, those short questions are also not following the law. The reason a math question is used, vs say a history question, is that math has no language barrier and the question should have 4 parts. eg. 100 x 2 / 4 + 50 - 25 = 75

And yes, we do not pay income tax on winnings of any kind.